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

Query syntax might be tough with a list-user field and user fields are always interesting. I would try the following: 

In your activity table make a new field that does: 

UserListToEmails([Team Members])

Then adjust your query as: 

var Text QUERY="{'new field id of the one above'.CT.'" & UserToEmail([Team Member]) & "'}";

Size(GetRecords($QUERY,[_DBID_ACTIVITIES]))

I prefer this method because emails are much easier to read/see