How to play missions online with a friend

I just bought the game with my gf and I'm trying to play with her online. But we found no way to do missions together.

We tried making a temporary posse (we cannot pay for a permanent one) and invited each other. We can see each other and roam together. But every time we try a mission (stranger or yellow ones) we get split up.

I don't care if we are against or teamed-up with other player, I just want to play with her. I read tricks here and there, but it seemed ways to manipulate matchmaking.
Is it not possible to just play coop mission reliably ?
Is the whole online mode broken for playing with friends ?

Solution 1:

Both free roam activities (trader resupply, sales, strangers, bounties, hideouts, free roam events, etc.) and instanced activities (story, moonshiner story, showdowns, etc.) can be done co-op. When you are both in the same posse (temporary and permanent both work) and the posse leader chooses to to start an activity, all posse members join.

You said you invited each other, but did you accept the invite afterwards? On PC, after receiving an invite, you can hold F1 to view and accept it. Otherwise, you can open your player menu and look at your invites there. If you have trouble with invites, you can also change your posse settings from the player menu to be "Open" instead of "invite only". The other player can then find your posse in the player menu join it. You can then change the posse back to "invite only" if you don't want other people to join.

Keep in mind that other people can't join a permanent posse if you don't have a camp setup. But that is not a problem for temporary posses.