I see what you are asking, and currently we do not surface that relationship. Although that sounds useful, so I'll be sure to pass on the use case!
When our department uses Looker we have an org table that maps our individual org members to a certain team ID. That way we can build a field that we can filter on (with User Attributes or otherwise) to get the team's data as a whole. If you have something similar in place that maps users emails to a team name / user group, then you could just make a user attribute based on that dimension.
Otherwise, your idea of compiling comma separated strings would work as well.