I am a member of the URC the United Reformed Church and though you said you knew about Presbyterians I thought I would give you the Continental Reformed Perspective (otherwise known as the Dutch Reformed Tradition).
We practice padeo-baptism and view baptism as a sign and seal of belonging to the covenant made with God through Christ Jesus. We do not believe that baptism cleanses us of sin, only the blood of Christ does that but the water (sprinkling) does symbolize that cleansing. Being baptized in our church does not make one a member but a member must be baptized before they can join the church. We view baptism as the new covenant's sign and seal exactly like circumcision was used to seal the Abrahamic covenant.
Our communion is likewise a sign and seal of our union and adoption with Christ. We believe Christ is present spiritually in our meal although we don't believe He is there bodily (Catholic or Lutheran). We believe and confess that the sign and seal are so closely related to what they symbolize that the Lord's Supper is a means of Grace given to us, along with the other sacrament, Baptism and the preaching of God's Word. There is debate within reformed circles as to the frequency of communion. I am of the belief that it should be every Lord's Day as practiced by the church in Acts.
Communion is closed to members in good standing of any church that is a true church having ecumenical relations with the URC. These would all be reformed or Presbyterian churches not counting such liberal churches such as the PCUSA or the RCA. Here is a link to the church membership we would allow to our table:
http://www.service-life.com/sysfiles/member/links/links.cfm?memberid=303&public=1
Our Classis which consists of our ruling elders guard our table. Usually there is an announcement in the order of worship as well as a pastoral announcement during the institution of communion that warns anyone who is not a member in good standing or who has not asked forgiveness of their sins to abstain from the eating of the meal as they would only eat condemnation upon themselves.
We are of course Calvinists though I prefer the term Bible believing Christians. As what Calvin commented on was only Scripture and none of the doctrines that are ascribed to him were his but they are God's. As far as the Perserverance of the Saints (what you stated as once saved always saved) that is a clear doctrine from scripture. (John 6:35-37, John 10:27-29, Romans 5:9, Romans 8:35, etc...)
There are no different views within our denomination. Except as far as I have already talked about like frequency of communion, order of the liturgy, etc... If you would like to know more about how we practice our faith or what our doctrines, confessions and beliefs are I would be happy to discuss them with you.