To limit user access to specific Ad Accounts, there is an option within the Teams page, where you can flag a user as 'Restricted' - that user will then only be able to see within Adpulse the Ad Accounts they are tagged against.
Only Owners can 'Restrict' users
Only Manager and Viewers can be 'restricted'
When 'Restricted' that user will then only be able to see the Ad Accounts they are tagged against
How to Restrict a user
Navigate to the Settings > Admin > Teams page
Using the triple dot menu edit the profile of a user
In the Profile modal tick the Restricted checkbox and hit save
That user will no only be able to the see the Ad Account they have been tagged against. How to assign Users and Tags to Ad Accounts