Hmm...
I'm against multi-user battles, but just thought of something reading the work 'Liberation' there... If you run up to an ally, you can use an 'assist' ability - either an attack on the field, a temporary power-up, or give/use a chip for them. This would decrease their busting level/EXP gain/whatever, (less so if you're 'grouped' with them) and possibly penalise you (Either cost some form of 'energy', or chip is unusable in next battle to 'recharge' or something)
Like a cross between Liberation 'assist' and the Spare-Navi 'assist' in BN5DS