Yeah, I have to agree with other posters. I’m the bass (the only one) in my parish’s choir, and I love singing, but I would never presume to invite myself to join another choir with whom I haven’t rehearsed. Even if I know the songs and the parts, they are in tune with each other and it’s awkward to have a new voice just randomly join. That said, when we have people from other choirs visit our parish, we will often invite them to join us. We’re also a tourist destination (historic California mission) but I haven’t known of anyone who actually requested to sing with us. At most, they’ve mentioned that they’re in a choir at such and such parish, and we will say, “come sing with us, if you’re comfortable.” We have an SATB choir (barely, like I said I’m the only bass singer) meaning we sing parts, but we don’t mind others joining us. We have one microphone, I forget what you call it but it’s the kind that’s meant to pick up the choir as a whole rather than emphasizing individual voices. Since you’re not an SATB choir, it might be easier to invite people to join, but it really depends on what your choir members feel about it, and the priest if he has any opinion on the matter.