Need JSON REST command that returns what groups a given user is a member of

I cannot see a JSON API command that will return what groups a given user is a member of.  This is quite useful for administration of users, and syncing such information between tools, but I cannot see any such command in the documentation.  Hopefully I’m just not seeing it and it can be pointed out to me.  If not, I would respectfully ask that such a command be considered for addition to the API soon.