Privileges in MS Teams

Hello all,

I manage a MS Teams room for a company that subscribes to MS Teams, and work with a customer from a separate company in that Teams room, whose company also subscribes to MS Teams. Recently, someone from my customer (a "guest" user in this particular team) mentioned that they cannot edit a shared Excel document in the Teams room. Team permissions for the external folks are all guests.  Can anyone comment on why my customer might not be able to edit files there? Alternately, is there any way we can invite users from other companies as "members" rather than "guests"?  Thank you in advance for any guidance here.

All the best,

Jon