They are attendees, and may be on the church attendance roll, but are not members of the church. When they give their Profession of Faith and are baptized it is often referred to as “joining the church.” They can fully participate in the services but are not to take The Lord’s Supper (communion). Membership can be moved by letter (the church where they previously were a member sending notice to the new church), or by Statement of Faith to affirm they have accepted Christ and were baptized from another SBC church. It depends on the individual church if this will be accepted from another denomination - some are not comfortable doing this but others will if the faith tradition has a similar doctrine of Believer’s Baptism.
Interesting. I've never been to an SBC service so I had no understanding of it. That actually doesn't sound too different from the rules for membership in a Catholic church, aside from the “reaching the age of accountability” clause before you're eligible for baptism. Of course Catholics have that too, except it's used for receiving the holy spirit at a confirmation ceremeony.