How to organise a multi-select list that allows for selection of both users and groups, where users can have multiple groups?

So at present, the system I'm developing has a multi-select list that categorises users by their group. For example, the select list options looked like this:

• Administrators
  • John Smith
  • Jane Smith
• Finance Team
  • Jordan Smith
  • Jeremiah Smith
  • Joe Smith

And the selected options allowed for a whole category as well as individual users, such as: "Administrators", "Jordan Smith", "Joe Smith".

The problem is, the system has now evolved so that users can have multiple groups. So rather than just being a member of Administrators or Finance Team, they could be a member of both.

The select list needs to still allow for the selection of both individual users and whole groups. I could replicate the individual users across multiple groups, so that they appear twice in the list, but then I'm concerned it will confuse the end-user as if they use the typeahead, the user will appear multiple times under different categories.