You can, from you Enterprise Back-Office, add or modify existing Back-Office and API users, as well as select their user profile.
To create a new user, open the Tools > Configurations > Back-Office User, click on the Actions button, then new Back-Office or API user, fill in the required fields, and hit Save.
[API User profiles all include the Ws mention for WebServices]
To modify an existing user (same menu), just fly over the field you wish to modify and click on the pencil icon to proceed with the modifications.
Users cannot be deleted for audit trail purposes. You however have the possibility to deactivate them (Active to Inactive in the Status field).