Add notice in the UI that reassignment is restricted to enterprise users

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The following discussion from !187395 (merged) should be addressed:

• @justin_ho started a discussion:

  Thought (non-blocking): We don't really have any indication in the UI when this is true (apart from the different selection of users). I think we should create a follow-up to add a small notice to users along the lines of "Reassignment on this group is currently limited to enterprise users" so it's a bit more obvious.