I set a main assist key for each character, and then don't use any "target=" items in my normal attack spells.

Example:

Party1 has no key set.

Party2 has "f" key set with the following:
/assist [target=<party1_name>, nodead, exists];

Party3 has "f" key set with the following:
/assist [target=<party1_name>, nodead, exists]; [target=<party2_name>, nodead, exists];

etc...

Doing it this way, sets your targets, and they stay until you assist again.
BUT, if you have all of them set to follow party1..they are all going to move back and forth with the sheep too...so you may want to break /follow along with this.