i do it once a month for university groups signing up on the night works well, as does signing up in advance. a google form works well for in-advance sign-ups. something to consider might be if you want people to give content warnings on their work; if yes, maybe circulate a list of common content warnings which you'd like them to give the standard in my city's poetry scene is each person gets 5 minutes or 2 poems, whichever they reach first is their limit i agree with @Hannah C.; people love a drink at an open mic