1) John the Baptist, of whom there are said to be believers who know only his baptism present in Ephesus in the 50s (cf. Acts 18:24-19:7). Technically a pre-Christian figure, but here associated with Christian believers, so we'll group him in with the rest.
2) John the Evangelist, author of the Gospel and Letters of John (and I am fully convinced that whoever wrote the Gospel also wrote the Letters). Irenaeus, Eusebius, et. al., locate this figure in Ephesus.
3) John the Seer, writer of the Book of Revelation, to which Ephesus and nearby cities are addressed, and about whom Irenaeus, Eusebius, et. al., say he went to Ephesus after being released from imprisonment on Patmos.4) John, son of Zebedee. Irenaeus, Eusebius, et. al., locate him in Ephesus, and frequently identify him with the Evangelist and less frequently with the Seer.
5) John the Elder, known to us from the writings of Papias. He seems to have been a follower of Jesus. He is sometimes identified with John, son of Zebedee, and sometimes as a distinct figure.
Let us begin with the Evangelist. If the Gospel and Letters are to be identified with a figure named "John" known to us independently, then John the Elder seems to be the best candidate. 2 John 1 and 3 John 3 explicitly refer to the author as "The Elder." Indeed, if the traditional attribution to "John" is taken seriously then what you have are explicitly two letters written by a man named John, the Elder. The question becomes whether this person could also be John, son of Zebedee, and of this matter I am dubious. The Papian data referring to John the Elder seems to clearly distinguish between him and the son of Zebedee. I think that there is also good reason to think that the author of John's Gospel was more at home in Jerusalem than the Galilee and quite probably moved in elite circles, which seems less likely to be an adequate description of the son of Zebedee. While I cannot rule out the possibility that the Elder and the son of Zebedee are the same person, I think this to be highly unlikely. I am close to fully convinced that the Gospels and Letters of John originated from in or around Ephesus, were written by the figure that we know elsewhere as John the Elder, and that this figure was not John, son of Zebedee.
That said, it seems to be highly unlikely that the author of the Gospel and Letters also wrote the Revelation. Strictly speaking it's not impossible: I am unaware of any study that has shown conclusively that common authorship is 100% excluded. But it does seem that in the Revelation we're moving in a sufficiently different conceptual world so as to make common authorship highly improbable. So, we have at least one other Christian leader named "John" operating in Ephesus during the first-century. At the same time, we have data that indicates that John, son of Zebedee, also ended his days in Ephesus. We could thus be dealing with as many as three Christian leaders in this city during the first-century. I think however that two is a much more likely number.
Now, the possibility of more than one leader in the same city named "John" is hardly incredible, as we know that this was one of the most common Jewish names of the time. And we also have Eusebius telling us that were two tombs to John in the city, and saying that one belonged to the Elder, and the other the son of Zebedee. What is crucial for my purposes is that early Christians apparently had some recollection that there were two prominent first-century leaders named John active in the city, but not of three. Sometimes they confused and fused them, but the crucial point is that they had no recollection of three such leaders named John. Given the strong recollection that both the Seer and the son of Zebedee were active in Ephesus (the former confirmed by Revelation itself) I want to suggest the following: John, son of Zebedee was John the Seer.
This actually makes a great deal of sense. If one reads Revelation alongside the Synoptics and the Johannine literature and then asks "To which of these bodies of literature is Revelation most closely affiliated?" one would most likely answer "Synoptics." I would suggest that this is a product of John's membership in the Twelve, whom I believe gave the most definitive shape to the Synoptic tradition. Meanwhile, I would argue that the Elder's distinct voice in the Gospel has to do with his own experiences as a follower of Jesus who was not one of the Twelve.
What about the Christians associated with John the Baptist? I would argue that Luke has made exactly the same sort of error that we would see in later generations of Christian writers: he has confused Johns. He's learned that Paul encountered Christians baptized by John in Ephesus. He mistakingly associates that John with the Baptist, when he should have associated him with the Elder or the son of Zebedee. If that is what happened, then my suspicion is that these were followers of the Elder: son of Zebedee seems too prominent for him to have made this error. If they were associated with a relatively unknown Christian leader than it seems to make better sense that Luke would assimilate that figure to the Baptist than if they were associated with one of the most prominent Christian leaders. If I am correct, then we have evidence of Johannine Christians in Ephesus already in the early-50s.