Issue:
When I try to add a private group via Mixed Resources > Add Resources in a plan, it does not show any results ('No results found'). Can you please help here?
Cause:
Private Groups can be added to plans only if you are part of these groups. Due to their restricted visibility, you need to be a group member in order to add/use the group in this Plan, regardless of whether you have a (Technical) Admin role.
Resolution:
Check if you are a member of the private group and join if you haven't yet.