Hi George, the most ideal way which I haven't done yet at my employment is to use the AD connection where
you can get those employees in the department you want and then use that as the data source instead of the
officeUsers full drop down. The last time I looked at the connector, it required an account that had AD access -
I didn't need that type of functionality so I stopped there. The other alternatives would be less dynamic and
less desireable - where you have an access list and then filter based on those displaynames you have.
You may be able to create a Flow that updates a table - so it's refreshed daily.
I would definitely start with the AD connector - from either PowerApps (this would be realtime) or Power Automate which would be a daily or scheduled update to dataverse or a list.
Hopefully this provides some ideas -