Hey,
I've now been studying with a couple of my friends for a few weeks. Some days it works, some days it doesn't.
I think the problem is that we didn't come with a clear plan right from the beginning.
It would work best if you made a very detailed plan, like the stydy times vs. time discussing the questions and topics with the group.
You might even want to study like you would alone, but then at the end of the day, discuss all the topics briefly with the group, and maybe do a summary of the topics with the group together.
We're still in the very start, but we made a plan, and after that it's been going much better!