I'm not sure. Glenn said it was his first time taking a group with him. I'm not sure why a group was assembled in the first place. Maybe Merle forced his way into going with them?

I really wish we could see what the Atlanta camp was like pre-Rick with Shane in charge.

 

Well I think they took anyone that could help grabbing "stuff and things" (^^). Merle definitely had the skills needed for that mission.
And if they didn't have done that choice, well there would'nt have been this whole arc

 

What people forget is the fact that, besides his drug addiction and his one-man leadership mindset, he is a military vet.. so he know how to fight and knows how to survive.. He could be very usefull, but a pain in the ass either.

 

I think we get a good idea of what it's like, about 3/4 through the first episode,
and the throughout 'guts' and 'frogs'. I get the idea he was a resounding voice but definitely not a leader like Rick turned out to be. Shane probably kept hope alive, helped maintain equality and peace in the group, and was a high chair in the decision making of where, when, and how the group chose to move.