First get the userID of the user, there are several ways to to this:
/people/username/your-user-name # may return 1234, I'll use it within the next call
Use your userID if you are not an admin to avoid forbidden errors.
Then get the streamIDs, the user may have created a lot of streams and added different places to different streams:
/people/1234/streams # may return multiple streams, for example 2,3,5,7
Iterate over all streams and filter for the group type:
/streams/2/associations?filter=type(group) # may return zero, one or more streams
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