When I used to plan group trips I learned early on I was not planning my trip, I was planning theirs. I soon learned to suggest routes based on the participants involved and find that still works pretty well.

Whenever new people were allowed to invite themselves, they were warned repeatedly about what awful miseries may await them to such a degree, that if they experienced anything close to what was described, they couldn't say it wasn't in the brochure. If they still wanted to go after I made it sound so horrible, I figured they either may have the right stuff or would be too proud to complain much. In any case, my enjoyment of the trip was no longer dependent on theirs.

New people trips were always summer basecamping type trips a day (or three) in. Anybody can hack that and daytrips can be enjoyable for those who might get bored. Even then, while packing to leave one of the early 'traveling' campsites, someone would invariably say: "Why don't we just stay here? This is great right here. Let's just stay here." More often than not, those words didn't come from newbies. That's why I refuse to plan anything beyond a rendezvous with that person these days. Those are very enjoyable for me when he actually shows up when and where expected.  :-/

The two most important things I've learned is there are talkers and there are doers. Always get the cash upfront from the talkers - AND make sure you get any gear you loaned them back at the take-out. Don't wait for the post trip party.