How do I report on projects based on user other groups/teams?

When we have project owners in training, we'd like to build a dashboard for the training manager to monitor. Our hope was to pull these in based on an other group, team, or role called 'Trainees'.

Their home group/team should be the actual team they're assigned to for correct functionality of our team dashboards, but we also want to be able to report using this secondary assignment or association.

We've tried groups, teams, roles, and a custom form, but can't figure out how to build this report. Is this possible to build based on an other home/team or a different user setting/field? Has anyone done anything similar in another way?