This is the exact same principe than individual, except you use a "for" loop taking all party members.
Many exemples exist through the code, just type "member" on eclipse search tool, you should find 2 cases, clan members, and party members. Just open files which talk of party, and search for "for" loops examples. You can search aswell from the //recall_party command, which uses same mechanism in roder to find all members of a party, (except you port them to you instead of buff them, but it's secondary).
You have to check first if the guy is on a party.
-> If yes (party != null, or activeChar party == true), for all party members, do the same action you will normally do at solo (if they're != null aswell, to avoid NPEs cases due to disconnections).
-> If no, do the solo action on the character himself.
Buffs haven't to be changed.
Feel free to post your try in order to get some help. If you're lazy, well ::) I'm not the good people to ask for patch'n'play.