If I accidentally delete a user, is it possible to reactivate that user?

Yes. When you delete a user, a pop-up will appear, asking whether to deactivate the user or delete the user forever. Deactivating the user will give you the option to reactivate the user. When searching for a deactivated user, the Delete column will show the user as Deactivated.

Clicking the Deactivated link will open a pop-up, asking whether to reactivate the user or delete the user forever. Reactivating the user will reactivate that user’s account. Deleting the user forever at any point will delete all the user’s data and will prevent the user from ever being reactivated.