How to create a user for managing existing domain in Plesk UI?

Question 

How to add an additional user (domain administrator) who will be able to manage domain and have access to Plesk web interface?

Answer

For Power User view:

1. Login to Plesk
2. Go to Users > Create User Account;
3. Fill in all the required fields, choose a user role (it determine what Plesk functionality is available to the user once they log in) from the drop-down list, choose which subscription will be accessible for this user from the drop-down list:

For Service Provider view:

1. Login to Plesk
2. Go to Subscriptions > example.com > Users tab > Create User Account:
   
   
3. Fill in all the required fields, choose a user role (it determine what Plesk functionality is available to the user once they log in) from the drop-down list, choose which subscription will be accessible for this user from the drop-down list:
   

Additional information

User Rules explained