The users within Dynamics 365 Online are managed within the Office 365 Admin Center.  In order to be an enabled user in Dynamics 365 Online, the user must have a Dynamics 365 license assigned in the Office 365 Admin Center.

The following steps are needed to add a create a user in Dynamics 365.

  1. Go to the Office 365 admin center,
  2. On the Home page, choose Add a user
  3. Fill in the information for the user.
  4. In the Product licenses section select the Dynamics 365 CRM license.
  5. Click Add when you are done to create the user.  ** When you create a user and assign a Dynamics 365 licenses in the Office 365 Admin Center, the user is also created in Dynamics 365 (online).  The synchronization process between the Office 365 Admin Center and Dynamics 365 (online) can take a few minutes to complete.
  6. Next, in Dynamics 365 a security role will need to be assigned to the user.   Open Dynamics 365 and go to Settings > Security > Users.   The new user should be shown in the Enabled users view.
  7. Open the user’s record. 
  8. Click Manage Roles
  9. In the Manage User Roles dialog box, select the security role or roles you want for the user or users, and then click OK.

Additional information can be found in Microsoft's Dynamics 365 help,