We have 2 MYLRs in this household, each owned by different people with their own Tesla accounts, but each of the other granted access to the other car.
I can confirm that the differences between the owner's and grantee's app access isn't very much.
The owner can manage drivers both on the main app menu and under "Security", the grantee has neither option.
The owner has a "Specs & Warranty" button, the grantee does not.
The owner has access to "Charge Stats" and electricity cost rate settings, the grantee does not.
The owner can make "Software Upgrades" (Accel Boost, EAP, FSD) purchases in "Upgrades", the grantee can't.