Solved!
Thanks to cmillerLCE and chrlie
get-App -mailbox mtestd01
WARNING: An unexpected error has occurred and a Watson dump is being generated: Object of type 'System.IO.MemoryStream' cannot be converted to type
'System.Byte[]'.
Object of type 'System.IO.MemoryStream' cannot be converted to type 'System.Byte[]'.
+ CategoryInfo : NotSpecified: (:) [Get-App], ArgumentException
+ FullyQualifiedErrorId : System.ArgumentException,Microsoft.Exchange.Management.Extension.GetApp
+ PSComputerName : exc00000123.ad.ad.com
Set-App -Identity 7a91e319-a65d-4ceb-909b-12203561dbf5 -UserList mtestd01 -OrganizationApp
Get-App -OrganizationApp -Identity 7a91e319-a65d-4ceb-909b-12203561dbf5 | fl DisplayName,ProviderName,UserList
DisplayName : Cisco Webex Scheduler
ProviderName : Cisco Webex Inc
UserList : {ad.ad.com/DU/Users/TestUsers/Dennis M(Test)}
Get-App -Mailbox mtestd01
DisplayName Enabled AppVersion
----------- ------- ----------
Enterprise Vault True 14.2.0.1226
Cisco Webex Scheduler False 41.7.1
Bing Maps True 1.1
My Templates True 1.0
Suggested Meetings True 1.1
Unsubscribe True 1.0
Action Items True 1.0
Set-App -Identity 7a91e319-a65d-4ceb-909b-12203561dbf5 -UserList $null -OrganizationApp
Get-App -OrganizationApp -Identity 7a91e319-a65d-4ceb-909b-12203561dbf5 | fl DisplayName,ProviderName,UserList
DisplayName : Cisco Webex Scheduler
ProviderName : Cisco Webex Inc
UserList : {}
Get-App -Mailbox mtestd01
DisplayName Enabled AppVersion
----------- ------- ----------
Enterprise Vault True 14.2.0.1226
Cisco Webex Scheduler False 41.7.1
Bing Maps True 1.1
My Templates True 1.0
Suggested Meetings True 1.1
Unsubscribe False 1.0
Action Items True 1.0