I had an engineering teacher in high school who by far was the best (in my high school) at assigning group projects.
Students have a tendency to want to work alone because that is the environment they are accustomed to. High school teaches kids how to work in a 20th century factory: stay in line, follow the rules, do your work and let other people do their work.
My engineering teacher wanted us to work as adults would: he assigned us brief guidelines, and our group was responsible for collaborating and producing something for him. For example, as the first project in the intro to engineering class, he started by showing us a lamp he made. Then he asked us how one could make 10,000 of them for as low cost and as easily as possible. We had to deliver an assembly process (to make the lamp), a parts lists, and a floor plan of the building we would theoretically have.
I think what mostly made it so good was the lack of formatting. Many kids didn't like it, you had to actually listen when he talked because he didn't hand out sheets reiterating what he just said. You had to use your best judgement with regards to making the product look as nice as possible, as apposed to following some guidelines. The class made you think, you couldn't just go from one step to the next and get the correct answer, you had to think for yourself and make up your own steps.
Grading was a struggle for him, especially because this was one of his first times teaching this class. You got a grade for the project (everyone in the group got the same grade), and you got a grades for small check ins to make sure you were actually doing stuff in your group.
Engineering is mostly about problem solving, so when asked questions he would often respond "That's your problem". He did so if people complained they were doing too much of the work, or if their group wasn't listening to them, etc. You can't learn how to work in a group if some higher power solves your communication problems for you.
Hope that helps, sorry for rambling.