When an account reaches the max plan limit, adding an existing user that is already within the plan count into another repository of the same account causes error:

"Not enough users on plan. The owner of this repository will need to upgrade their plan before this action can be completed."

Steps to reproduce:

1. Have an account that reached the quota of the plan limit - say Plan limit: 5 / 5
2. Add an existing user that is within the 5 user plan count into another repository within the same account.
3. Error says "Not enough users on plan"

Expected behavior:
The existing user can be added into any amount of repositories within the same account.

The same issue can be reproduced with User groups.
I.e.:

1. Have an account that reached the quota of the plan limit - say Plan limit: 5 / 5
2. Add an existing user group that is within the 5 user plan count into another repository
   within the same account.
3. Error says "Not enough users on plan"

Additionally, adding an empty User group to a repo will give the same error.

Notes:

At the moment, the issue occurs when trying to add an existing team member from the page of the specific 'User Group', e.g. https://bitbucket.org/account/user/TEAM_USERNAME/groups/administrators/

Workaround:

1. Open the 'User Groups' page of the team/account: https://bitbucket.org/account/user/TEAM_USERNAME/groups/
2. Select the Add Member button on the top right corner

You'll be able to add an existing member to the team this way.