If you are trying to modify a planner or add/remove planning partners for a cycle's planning group, the person you are looking to add must be a user in the Pequity tool. If they are not appearing in the dropdown, you will need to add them as a Pequity user for them to appear.
When adding the new user, you may provision them a "Guest" account. When moved to the Set Permissions screen, you may leave their geo, department, level, and range permissions blank - these determine their permissions for the Ranges & Offers experience. Their comp cycle permissions will follow the cycle, so these are not applicable when it comes to cycle access.