The BFM2000 says of baptism (Article VII), "Being a church ordinance, it [baptism] is prerequisite to the privileges of church membership and to the Lord's Supper." [underlined added]
My guess is "the church ordinance" refers to water baptism as a prerequisite to membership in the local assemble. Leaving aside the issue of whether water baptism is a prerequisite of salvation, it is not a prerequisite of taking the elements in remembrance of Him. Anyone professing they believe in Christ, and thus have been spiritually "baptized" into Christ may partake in His remembrance.
Examine yourselves, to see whether you are in the faith. Test yourselves. Or do you not realize this about yourselves, that Jesus Christ is in you?—unless indeed you fail to meet the test!
God did not say for us to examine visitors to see if their papers are in order.