It happens, so be prepared! This is your consolidation test. LOL
Step 1: I come back with, “I will play if you sing.” That either ends it right there or makes it a lot of fun (at least for me). You run the risk of boring everyone to tears and clearing out the room if you force “Mad World” or whatever else you can play/sing by rote.
Step 2: I’ve got a willing singer! It might be a guitar that I’ve never even seen or held before, so I have to get it in tune and get a feel for it. Can you get a guitar in tune without a tuner? Others might need a tuner app on their phone but you can also take the time to learn how to tune a guitar by ear. Just sayin’.
Step 3: If I have a willing female singer I will suggest “Dreams” by Fleetwod Mac. – two chords and they can unleash their inner Stevie. From there “What’s going on” works well – easy progression --I usually need a quick refresher/glance at the progression from the Googler.
Step 4: Play for the group, take requests. Play what THEY are willing to sing. Pull up the chords on the phone, try and suss it out. Lather, rinse, repeat – and don’t forget to stay hydrated. 
EDIT: Go to songs are for open mics and bowling alley gigs (and such).