We do a similar thing, once a month in the St Johns hall in town, a rythmn, lead, and bass guitar. The locals, some in their eightys come and sit and just listen, after a couple of hours we knock off for a cup of tea, then carry on for another hour and a half.
We use a PA system for the singers and indivual amps for the musos, if we want to reduce set up time, the bass amp has 3 inputs and can handle singer and bass together.
Walk ups is fun, you never know what the next singer will do.
Keep going and have fun.