How to add friends to a game?

Hey,

so I have a little issue with the DE, had the same in beta already. I have a friend on that xbox app i would like to add to a game i opened. So we are both online, I click on "invite" and that xbox window open up. I see them as offline (although everyone is online) and sent an invite nevertheless, but nothing happens. they get no invitation at all (I suppose it would send a notification then, much like in Steam?) and I still see them as offline. Am I missing something or is this a bug?