The most likely reason for seeing this error is because users cannot have multiple points of access to the same piece of equipment.
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As such, if a user has been assigned Equipment X on their individual profile and you are trying to add them to Group 1, which also contains Equipment X, you will get an error message saying you are not able to add the user to the group.
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If you want to add the user to Group 1, you either need to remove Equipment X from their individual profile or from the group. Please note, if removed from the group, it will remove access for all of the users assigned to that group as well.