If it is really "open mic" i.e. with PA, an acoustic guitar in the front row will hardly be heard.

If it is a singaround (sinagrounds are so much more fun) singers' club or music session then I do think not only that joining in is OK but also that it often adds hugely to the sound.

But if you have a paid guest then even added vocals on verses (as distinct from choruses) may be thought pushy, and mostly instrumental joining in would be intrusive unless requested