The other students suddenly realize that there's no one to work the problems for them, so they also choose to work individually. The calculus group projects which were collected were graded with each group member receiving the same grade. Students were content with this. In the math ed classes, they were so used to group work that they complained about being tested individually but accepted the reason that we needed some way to assess them individually. This is optional, but has a very high participation rate.
Students appreciate having the group test prior to the individual test.
The above answers are for calculus courses only. It builds rapport between students and between the students and me. I often sit down for a few minutes with each group. After I get to know them, the lectures become more free-wheeling and they interrupt to ask questions when they cannot follow. Major projects constitute a significant part of the grade in some cases.