if you sing well (this mostly means that you know the tune), and at a 'regular' pace, and do songs that are known to the group, and don't mind folks playing along, ask at the beginning for a 'C' or 'A'..etc....but if you sing 'freely' and in odd keys, best make it clear that trying to accompany you might be a problem.

Out local circle has quite a number of singers who use NO instrument, or often sing without one, and I would MUCH rather hear them unaccompanied than to watch the struggle that often results when several instrumentalists spend ¾ of the song figuring out what what to do. "Sweet Betsy from Pike" is easy to play along with, "Molly Brannigan" isn't unless they have practiced, or are very quick.