Group lessons are definitely the way to go.
I've always taken group lessons, all throughout my riding. :) I have thought about going to private lessons, but I'd miss my friends too much.
LOL! Plus, it's good to get your horse used to being around other horses, & some even behave better than when they're alone in the ring. :P Although, there's nothing wrong with private lessons. They are good for if you really need individual attention, & if your horse does as well.
In my group lessons, there are about 4-5 of us- we do get a lot of individual attention, despite that we're in a group. It depends on the trainer(s) if they will give everyone individual attention or not if you're in a group. At my other barn they did the opposite- just talked to everyone at the same time, & not really helping them individually on what they need to work on. :) So, I say go for the group lessons!