Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Hi Hugh,
This is not an option that is offered by Microsoft Planner. A plan is bound to an Office 365 group. Adding a user to a plan means adding him/her to the group.
If you do not want the user to have access to other plans in the group, we suggest creating a new plan with a separate members.
We do understand some situation where these requirement arises, and we think it might be a good idea to have this option in the future, so we also suggest you to provide feedback to our Planner UserVoice. This will allow other users who have the same request to add a vote, increasing impact to the request.
Regards,
Sheen