Please, firstly make sure that user (that currently is inactive) processes the "Enabled" status, and the type of user does not match the following expressions:
2: Support User;
3 : Non-interactive.
If everything is fine, follow up these steps:
- Sign in to the Office 365
- Click Users > Active users and select the user.
- Under Product licenses, click Edit.
- Turn on a **Dynamics 365 for Customer Engagement apps ** license, and then click Save > Close.
- In the Office 365 Admin Center, click Groups > Groups.
- Choose the security group that is associated with your Customer Engagement apps organization.
- Under Members, click Edit, and then Add members. Select from the list of users with Office 365 licenses or use Search to find users.
- Select the users to add to the security group, and then click Save > Close multiple times.