As the main account owner, you may eventually run into the issue of not being able to edit certain fields within a sub-account's contact record. Once a sub-account owner accepts the invite and creates their account, the sub-account contact record will be locked and you will no longer be able to edit most fields from your main account.
However, this design is intentional. The primary reason for this is that a sub-account owner will set up their profile within their sub-account. Your main account then takes that same data from their account and syncs it into the contact record you have for the sub-account. This helps to maintain consistency throughout both accounts and will avoid potential issues that might otherwise occur.
The secondary reason is to allow sub-account owners to possess a level of autonomy within their accounts. While we place far greater restrictions on what sub-accounts can and cannot access, they will always be able to modify their profile as necessary. If for any reason a change needs to be made to their profile, they will need to execute it from within their sub-account.
With that said, you will still be able to edit certain fields for both a People contact record and a Business contact record even after the lock is in effect.
The fields you will continue to be able to edit include:
Note - Even after a record is locked, you will still always be able to use other aspects of the contact card. This includes Notes, Reminders, Activities, InstaSites, InstaReports, and any other project management or communication actions.