To enable an existing user or a bunch of users login to the MSPControl and click on Users (User Organizations),
or go to Organization > Organizations > Users.
You will see the list of all users. If you have many, it might be a good idea to use Search form in the right to find the one you need. You can choose whether you want to search by Display Name, E-mail Address, Domain Account, Account Number, or Login.
Disabled users have a red cross, while enabled ones have a green check mark.
Once you found the user(s) you were looking for, you have two ways to disable them.
First, you can open user's page and enable him there. To do so, click on the user in the list.
Scroll down and un-check Disable User.
Scroll down again and click Save Changes or Save Changes and Exit. The former will leave you in user's profile, the latter will redirect you to the list of all users.
Secondly, you can perform the same action - enable one or several users - from the list of all users. Select them by checking the corresponding check-boxes in the left.
Then select Enable in the drop-down menu Actions, and click Apply.
You will see a notification Action has been successfully performed.