To activate one, or more users, first navigate to Admin > Organisation > Users.
Search for individual users in the Search field or select Inactive users in the left hand menu filter. Checkmark one, or more inactive users and click the icon Activate selected users.
In the left hand menu, you can also search for, and filter by entire departments to activate all or some of its inactive users.
Bonus info: Above the Create new user button, you can change the setting from Show 25 per page, to display a larger number of users in the user list. This will speed up your process of activation.
The final step is specifying which user activation method you want to apply. The three methods are described in User Activation.