Hi there Ecarlette, welcome to dA!
Groups are basically a conglomeration of artists who are bound by a common interest. In the case of Infinite (this group), it's for artists who are drawing for a charity artbook to help build a library in rural China.
The benefits for joining a group are that you can make some nice online friends, and improve your art by getting to know other people who share similar interests! It's also a great place to find out more about the community, and be a part of some cool projects. Feel free to look around on dA for something that catches your eye!