direct_add support for sub-groups #api

Srikanth Hanagud

It looks like we currently have a way to direct add members to subgroups but only if the user is not already in the main group. We can use the subgroupIDs to pass the sub-groups the member should be added to.
However, we’d like to be able to direct add to subgroups after a user has been added to the main group and there is no way to do this right now. 
If we try using direct add when the member is already added to the main group, we get a direct_add_existing error.

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