From the Home screen, add a tile showing all project members, their company, roles and email.
This gives all team members a point of contact for coordination and lets all teams know if they need to bring in additional members. Obviously this tile should be visible by choice if a project has teams/companies that Admin may not want made aware of.
As an account Admin I can view all members of a project, however my company (an MEP firm) often gets invited to collaborate on the Architects BIM 360 site. A lot of time is spent requesting project access for members that may already have access, a contact list would change that. Also just knowing who i can delegate tasks to based on who has access.