Good morning cmsherro,
If I understand correctly, you are trying to extend the Org Browser app so you can count the number of users associated to the department of the selected user, or just simply get a count of all users in a particular department.
You may have noticed the count of Direct Reports on the second screen, with hopes that you can expand upon that. It's true at first glance that formula looks promising:
"Direct reports " & "(" & CountRows(Office365Users.DirectReports(First(Employee).Id)) & ")"
However, Office365Users. intellisense will not go directly to "Departments" as it does with "DirectReports" although it will return the department of a selected/identified user. See this reference:
https://powerapps.microsoft.com/en-us/tutorials/connection-office365-users/
I was able to count the results of a SearchUser formula, but the search user formula only allows for search of display name, given name, surname, mail, mail nickname, and user principal name. So I don't believe it's possible to count all users in the Marketing Department unless their email alias, or mail nickname, could give that away. Azure Functions / Graph API may be the way to go for your goal - but I have not tried that yet.

If you believe there is a use case for this please post an idea here:
https://powerusers.microsoft.com/t5/PowerApps-Ideas/idb-p/PowerAppsIdeas
Audrie
P.S. There have been reported issues of the org browser not working on the phone. I will look into that and see if I can learn why that is occurring for you.