How do you assign App/Desktop Groups to a AD group instead of individuals users?

Regarding management of the desktop pool, Microsoft has a mgmt module available on their Github page:

https://github.com/Azure/RDS-Templates/tree/master/wvd-templates/wvd-management-ux

For adding/removing users, PowerShell is your friend (until the management page is released), written a short script for you based on the cmdlet here:
https://docs.microsoft.com/en-us/powershell/module/windowsvirtualdesktop/add-rdsappgroupuser

Add-RDsAccount -DeploymentUrl "https://rdbroker.wvd.microsoft.com"
$Users = Get-Content .\Users.txt
$WVDTenantName = "Type your tenant name here"
$HostPoolName = "Type your host pool name here"
$DesktopUserGroup = "Type your Desktop user group name here"
Foreach($User in $Users){
Add-RdsAppGroupUser -TenantName $WVDTenantName -HostPoolName $HostPoolName -AppGroupName $DesktopUserGroup -UserPrincipalName $User
}