I just tried to play online with a friend. We tried inviting each other though both Steam and Discord, and could not join each other’s games. In halo, on the squad list there would be a blank slot reading “offline”. This is clearly an issue with Infinite and once again 343 needs to get their crap together and get the basic functionality working!
I suggest you submit a ticket on the Halo Support site for this
https://support.halowaypoint.com/hc/en-us/articles/360036218972-How-to-Submit-a-Halo-Support-Ticket