Moderate-sized groups - you have much more chance of having your errors corrected, even by non-native speakers. There is also much greater stimulation.

I think the best way to learn any language is in group because we can get partners to practice. Just like what people say, the more we pratice,the better we will be. Moreover, we can help each other out in group and somehow, others can motivate and give valuable feedbacks. But some people would prefer to learn it individually, probably because it's not noisy so they can concentrate well and they are totally free to choose the materials to meet their interests and skill.

Personally, I think individually is the best. I help many with corrections, as you can see in my profile. Also, if you use either qq, or skype, that will help as well.

What about a mix? They mostly say groups, as langauge is a synonym for "communication". Not quite my personal preferrence, though.