Co-Op Campaign Help/Advice Needed

I want to play the campaign on multiplayer co-op, but my friend doesn’t show up in my Roster or my game despite joining the Party. How do I do start the co-op multiplayer campaigns? What I’m doing is this:

Main Menu -> Halo: CE-> 1) Select Missions -> Open Roster (X) -> Invite Friends or 2) Main Menu -> Open Roster (X) -> Invite Friends.

I’m also having trouble joining his game, too. First it said Privacy settings, but then it just said it wasn’t able to join at the moment and to try again later. What’s going on? Can I fix it?

Sometimes it’s an xbox live problem I noticed, where I can’t see any friends, but they can see me online. Other times it is a MCC problem and rather annoying either way. You can open roster at any point to invite someone to your party, so it shouldn’t matter at which menu you’re at.

So after I send the invite, what’s SUPPOSED to happen? I’m supposed to see him right beneath my Gamertag and whenever I go to play the campaign he should come with me, right? AKA, everything should start at the same time?