All you need to know is here and
here.
Technically, the GMRS is for family communication, but in practice I think a person who has a license can blanket that license over any group that person is currently part of, such as your party.
If you don't want to pay the license fee, then buy FRS radios and forget GMRS. Think of it like machine guns and tax stamps.
I do not advocate using licensed services without being licensed, but in real life, the FCC probably won't care, UNLESS you interfere with a licensed station or repeater system. You might interfere with licensed users or repeaters even without knowing that you're interfereing. Individual licensees may care, and may look to record your transmissions, collect other data and report you. Be warned that if you DO irritate someone, and get reported, or if the FCC happens to be listening that day in that area (very low probability of that), you could wind up with some very heavy fines, plus forfeiture of your radio equipment.
Jim