OTbase Inventory features an internal user database. However, if you can you will want to use an existing Active Directory as your means of authentication. Both is possible.
You can access the local user database by selecting USERS from the main menu and clicking USERS.
In order to add a user, click on Add and work through the various fields.
This is not necessary if you authenticate using Active Directory -- no local users need to be created.
All authorization happens within user groups. You don't set access levels for an individual user, but for a user group. (If you had to set specific access rights for an individual user, you would create a user group specifically for that one user.)
If you authenticate via LDAP, you create a user group that matches an existing group in Active Directory and then check the LDAP Group box. That's it.
You can learn more about users and access rights here.