Organize well and make sure everyone's expectations are very clearly defined within the group.
We brought a late teens babysitter with us on our trip and it was disastrous. She'd always behaved reasonably in previous professional contexts, but something about Disney World triggered a reversion to very immature behaviors that seriously impaired our vacation. We regretted not writing a more formal contract and requiring a partial bond.
Obviously, the context in our case was different, but I think it's still important to establish protocols and expectations for fundamental behaviors and purposes (i.e., "please remember this is an event to celebrate DING's birthday"), and more specific occurrences like touring, dispute resolution, etc. Travel can be high stress and even people you know well can exhibit unexpected behaviors.
Also, don't rely on cell phones for managing split groups. The networks are busy at WDW and occasionally wig out. The park is loud and calls are easily missed over the clamor of the crowd. We found ourselves wanting walkie-talkies. Fallback meeting places and contact methods are important to establish.