While being in a lounge is usually better than waiting with a bunch of people at the gate, the lounges at EWR (business or Senator) and the ones at FRA that you have access to (so not the FCT or FCLs) are nothing to write home about so I wouldn't be too bummed about not being able to squeeze all your guests in. You're better off getting to EWR without too much time before your flight and sitting at a decent bar/restaurant and in FRA doing some (window) shopping and perhaps grab an Apfelstrudel!