It is better to have department information for user account in ArcGIS portal. A lot of times we need to get all users information, which need to know the user belong to which department.
Would setting up Member Categories work for you?
https://enterprise.arcgis.com/en/portal/11.1/administer/windows/manage-members.htm#:~:text=own%20ema...
You can also try using admin groups that will group members based on their department group in Active Directory, LDAP, SAML, etc.
https://enterprise.arcgis.com/en/portal/latest/use/create-groups.htm#:~:text=to%20the%20group.-,Link...
