Most of the sessions I go to are mixed, i.e. songs and tunes, and songs may be accompanied or not (preferably as the singer prefers!) We advertise the sessions as being "songs and tunes": most, but not all, of the instrumentalists who come enjoy the songs, whether they themselves sing or not, otherwise they'd be off to a "tunes only" session.So "Songs and Tunes" or even "Tunes and Songs" would be my preference for promoting these sessions.
I am equally happy singing a song I know unaccompanied, if it is a strictly "unaccompanied singing" session, though I have come across people who can NOT sing without their accompanying instrument to hold them on key or get the support of the riffs between lines of the song to hold it together. (Ever noticed how some unaccompanied singers shorten the long notes at the end of lines?) And I would challenge any notion that singers are not musicians when you hear some of the amazing harmonies that erupt spontaneously in such sessions.
I don't object violently to the "Singers and Musicians" label, tho' I'd say we are ALL musicians of sorts (even if not professionally trained, JJ).