Until recently, the suggestion that members of the early Christian congregations may have simultaneously been members in other associations and guilds remained under-explored. In Associations, Synagogues, and Congregations (click the Books / Articles tab for a free download), I dealt with the question of multiple memberships in connection with the Christians at Corinth (addressed by Paul in 1 Cor 8 and 10) who were attending banquets alongside non-Christians (“pagans”). I also considered the possibility that the opponents addressed by John’s Apocalypse, especially those accused of eating idol-food (or idol-meat) with “Jezebel”, may have been encountering sacrificial food as members in the guilds of Thyatira (something that William Ramsay suggested, but did not explore, long ago).
In a more recent article dealing with Sardis and Smyrna, which you can read on my publications page, I looked at the implications of multiple memberships for questions of rivalries and competition among different groups.
I have now just read a very interesting article on associations on the island of Rhodes by Vincent Gabrielson, which drew my attention to another interesting case of multiple memberships in associations (dealing with IG XII 155). A man named Dionysodoros, who was an immigrant from Alexandria (in Egypt), was honoured by a number of associations (koina) at Rhodes in the second century BCE, including the “Haliasts and Haliads,” the “Paniasts,” and the “Dionysiasts” (devoted to the god Dionysos). A closer look at this lengthy inscription shows that he was not only honoured by these groups, but was also a member in at least four associations at Rhodes! (See Vincent Gabrielson, “The Rhodian Associations Honouring Dionysodoros from Alexandria, ” Classica et mediaevalia 45  137-60.)
And these memberships were not fleeting. Dionysodoros was a member of the “Haliasts and Haliads” for 35 years, and he acted as their chief-of-banquets (archeranistas) for 23 years. Simultaneously he was a faithful member and benefactor of other associations, including the “Paniasts” whom he served as chief-of-banquets for at least 18 years. This is the sort of atmosphere of multiple affiliations and interactions in which the early Christians and diaspora Jews found themselves. So we should not be too surprised if we find some Jews or Christians going to synagogue or church one day, and hanging out with friends in the guild or association the next.