Report showing how many times a user appears in a list-user field

When you make a summary report any underlying detail record can only appear once. This applies to summary reports and charts and in fact it applies to table reports as well. A record can only appear in one place on the report.  

So the low tech solution to your question is to put a Dynamic Filter on the report  for the List User field conveniently it should let you select single individual team members and have the report instantly filter to get the answer for that one team member. But of course it does mean you have to one by one click on the different dynamic filter choices to get your totals.  

The only other way to do this would be probably more work than you care to do. You would need to have a pipeline say run each night and wipe out and create a set of children for each activity team with the individual team members as child records. So if you had three users in the list user field that activity team would end up with three children. Then you could make a perfect report. But now you've got a build a pipeline so I may not be worth it.