Within InterviewPlanner, we import interviewers from your connected directory system. If you're using Microsoft 365, we don't import unlicensed users. This means that if you intend on utilizing a user as an interviewer, you'll need to make sure that they have a Microsoft license associated with them.
This usually shouldn't cause any issues since a license is required to have a valid email address within a work or school Microsoft account. This means that it's very likely that every Microsoft user that is owned by an individual will have a license.
However, if you are seeing that you're missing some interviewers, this might be the cause. Check with your Microsoft 365 administrator to confirm that the missing user has a license.