A few years ago, I was very active with a local outdoor group that has since migrated to meetup.
I made a few stumbles when I organized trips and learned my lessons after a few hiccups.
Out of the experience, I learned the following
1) "Social" trips. Usually in the local foothills that are easy hikes. Full moon and/or food based. I would cap the trip limit, but would generally let anyone on them. If I noticed someone struggling on the trip, I would be helpful but they probably would not be on the more difficult trips. :)
2) Medium hikes. 10 miles or so (or less) with moderate elevation gain. If I did not know them, asked by e-mail if they hiked before. Some people of course exaggerated their experience and skill level(usually men. :D) and slowed us down. One young woman struggled and I ended up carrying her pack for the last couple of miles. Which leads us to:
3) Long hikes. 15 or even 20+ miles with elevation gain. Often off-trail. I screened these trips thoroughly. If I did not know the person, they were asked about their experience level and if asked they could tell of trips they did before. I flat out told people "NO" at times. The young lady above was quite upset when, after I carried her pack part of the way on a previous trip, I told her "No" on another trip. One gentlemen, flying in from *sea level* the day before, heard about my trip from a friend (16 miles, 3500' gain...and at ~12k elev IIRC) . When I again said "No", he became rather irate. :)
I rarely organized overnight trips as that was another ball of wax. Too many variables!
I did organize a beginners backpacking trip a few times, but I knew up front the nature of the trip/people on it. Being a beginners trips, people were very honest about their experience and we went through everything (gear, rout planning, and so on) thoroughly together. Never had any trouble on those trips.
So, all depends on the groups and the trip leader.
I now have a core group of friends and have not participated in "official" group trips in about 4 yrs. About the same time I met my now wife on a trip I organized. ;D