In my opinion the best and easiest way to get this achievement is doing the following steps:
1. Play one match in any mode (You do not have to complete the match, you can leave)
button on controller go to --> people and choose "Recently played with", add any guy from the list.
3. In main menu of halo 5 press
and find the guy you have added, press
and you are able to choose "spectate"
That is all :)