Question
How to add an additional user with access to Plesk interface and ability to manage all subscriptions or a particular one?
Answer
Go to:
for Service Provider view:
Subscriptions > example.com > Users tab and click Create User Account.
for Power User view:
Users and click Create User Account.
Fill in all the fields:
select a user role, which determines what Plesk functionality will be available to this user
select which subscription will be available for managing for this user (all or specific one)