Your problem is that the proper context for this meeting does not begin in verse 16 but all the way back in verse 5 which is more inclusive than the "eleven":
5 And the angel answered and said unto the women, Fear not ye: for I know that ye seek Jesus, which was crucified.
6 He is not here: for he is risen, as he said. Come, see the place where the Lord lay.
7 And go quickly, and tell his disciples that he is risen from the dead; and, behold, he goeth before you into Galilee; there shall ye see him: lo, I have told you.
8 And they departed quickly from the sepulchre with fear and great joy; and did run to bring his disciples word.
9 And as they went to tell his disciples, behold, Jesus met them, saying, All hail. And they came and held him by the feet, and worshipped him.
10 Then said Jesus unto them, Be not afraid: go tell my brethren that they go into Galilee, and there shall they see me.......16 ¶ Then the eleven disciples went away into Galilee, into a mountain where Jesus had appointed them.
In this passage of scripture, there are three groups of his followers listed that were to meet him in Galilee at an appointed place, later identified in this same passage as that mount. (1) "The women;" (2) "his disciples" (3) "his brethren." This is the very same division Luke gives in Acts 1:13-14 of the church in Jerusalem.
13 And when they were come in, they went up into an upper room, where abode both Peter, and James, and John, and Andrew, Philip, and Thomas, Bartholomew, and Matthew, James the son of Alphaeus, and Simon Zelotes, and Judas the brother of James.
14 These all continued with one accord in prayer and supplication, with the women, and Mary the mother of Jesus, and with his brethren.
These are the same ones who continued stedfastly to meet in Acts 2:1 later called "the church" in Jerusalem (Acts 2:47).
Additionally, Matthew 28:17 says "some doubted" which cannot refer to the "eleven" as Jesus appeared to the eleven twice in Jerusalem before going to Galilee for the purpose to remove all doubt remaining in them with Thomas. Moreover, the women could not be those who doubted as angels appeared to them followed by the appearance of Jesus to them prior to this Galilee appointed place. The only possible ones who could still doubt are "the brethren" in addition to the women and the eleven as there is no recorded appearance unto "the brethren" prior to this Galilee appointment. These "brethren" must have included those considered to replace Judas in Acts 1:13-22 as the replacements had to be eye witnesses of his resurrection (Acts 1:21-22).
Moreover, this is the only location provided in the gospel accounts where "over five hundred brethren" saw him "at once" (1 Cor. 15:6).
Therefore, to presume that only the eleven were present is simply ignorance of the full context of this event. Moreover, Jesus has already stated that final administrative authority is in the church "tell it to the church" (Mt. 18:17) rather than "tell it to the apostles" or "tell it to the elders"! Moreover, this is the usual manner of Christ when speaking to his churches to address them through its leadership (Rev. 2-3) unless you believe John sent his letters to the churches by "airmail" (literal "angels") but rather he addressed "the churches" through their ordained messengers - or leaders. This is precisely the same manner in Matthew 28:16-20 he would address the church at Jerusalem. And there was a church already in Jerusalem or else he could not have previously said "tell it to the church" (Mt. 18:17).
Finally, the apostles acknowledge they are subject to church authority. It is the church at Jerusalem that sent apostles to Samaria (Acts 8:14). Peter brought several brethren as witnesses to the house of Cornelius as he realized he would have to give an account to the church for his actions (Acts 11:1-17). It is the "church" that sends Barnabas to Antioch rather than the apostles (Acts 11:22). It is the church at Antioch that sent out Paul to the mission field (Acts 14:1-3) and returned to this church after every mission. Even though Christ had commissioned him personally, the Holy Spirit sent him through the church. Paul did not take off on his own apart from submitting to and working under the authority of the church at Antioch. It is the church that sends Paul and Barnabas to the Jerusalem counsel (Acts 15:1-3). The one sending always has more authority than the ones being sent. It is not merely James the apostle or the other apostles that determined to send out men but it was the action "with the whole church" (Acts 15:22). In Revelation 2-3 Jesus does not tell the Pastor to enforce his will and exercise discipline over members who resist, but rather addresses "the churches" to hear and obey. The apostles were "set in the church" (1 Cor. 12:28) and so our ancient Baptists when directly asked where does authority lie between the ordained leadership or the church, they responded that the authority lies with the church with its leadership, as they did not view the leadership separate from the church but IN the church.
From a secular Baptist history perspective, the earliest records of English Baptist on the subject of the keys of the kingdom are as follows:
In the Associational records of the early English Particular Baptists in 1655 it was asked if the authority symbolized by the giving of the keys was given to the ministry or to the church.
“Query 1. Whether the power of the keys spoken of in Mat. 16:19, John 20:23, Mat. 18:18, be given to the church or to the eldership in the church?
Answer: the exercise of the power of Christ in a church having officers, in opening, and shutting, in receiving in, and casting out, belongs to the church with its eldership, Mat. 18:17f., I Cor. 5:4., III John 9ff., Acts 15:4,22” – B.R. White, ed.,Association Records of the Particular Baptists of England, Wales and Ireland to 1660. (Association Records of the West Country, 1655), p. 60.
When they were asked about whether it was proper for ministers to go forth under some authority other than the church they replied:
“Answer: it is unlawful. 1. Because our Lord Christ sendeth forth his ministers by his power alone, Mt. 28:19, and hee is the head of the body the Church that in all things hee might have the preheminence, Col. 1:18; Eph. 1:22
2. Because Christ hath left ALL POWER IN HIS CHURCH both to call and send forth ministers, Mt. 28:19-20, saying, I am with you to the ende of the worlde, and I. Tim. 3; Titus 1; Acts 14; Mt. 18 and 16:18f.
3. Because wee finde the Church ONLY exercising that power both in chusing and sending forth ministers as appeareth by these Scriptures, Acts 1:23,26; 8:14; 13:2f and 11.22. Wee think fitt to adde that wee taking this question intire consider it fully answered.” – B.R. White, ed., Association Records of the Particular Baptists of England, Wales and Ireland to 1660. (Association Records of the Midlands, October 1655) p. 23, (Emphasis mine).
When asked if an ordained member of the church could just go out on his own accord to preach the gospel without being church sent they responded:
“Answer: we answere that such a brother soe judged of by the church ought wholly to be at its disposing. First, because that all those gifted are the church’s, I Cor. 3:22; 12:28; Eph. 4:11. Secondly, because if one brother goe forth at his owne will, then another and so a third, and by that meanes the church may be wholly neglected. Thirdly, because, if such a brother miscarry in his ministerie, it would be charged upon the church, and soe it would prove very dishonorable to the church and truth of Christ. Fourthly, because, in such a disorderly going out, he cannot expect the prayers of the church for the Spirit of God to accompany him, Col. 4:3; Eph. 6:18f, and wee judge if any brother shall persist in such disorderly practice after admonition that it is the church’s duty to deale with him as an offender.” B.R. White, ed., Association Records of the Particular Baptists of England, Wales and Ireland to 1660. (Association Records of the West Country, 1657) p. 34.