Allow user groups in meeting attendee fields

Allow meeting attendee fields on meeting notes to support selecting user groups, not just individual users.

This would make it easier to manage recurring participants, keep attendee lists current as teams change, and reduce manual updates on each note.

Expected behavior:

• Meeting attendee fields can reference one or more user groups.

• Group members are treated as attendees for display and downstream workflows.

• Teams can still add or remove individual attendees when needed.