How do you join each others missions?

[NorfolkDave]

Started playing a couple of days ago with a mate.
We sussed how to team up
We sussed how to get on the same world (that was an wierd one)
We sussed making a hive
We sussed (with help) how allow building in each others bases.

However despite having the same missions we can not seem to help each other, its like were playing a single player game but at the same time. Even as far as my mate being teleported to his own "dungeon" to face a boss by himself despite both of us needing to kill it and both of us being at the same spot at the same time.
I am guessing we have to do something to sync the quests? Were playing PVE as we like to game together and both hate the toxicity of pvp in games like this. I do wonder if we are still on the tutorial stage and the missions are meant to be solo,....but were a few hours in now

[j.bonaiuto]

I believe that first big deviant that is triggered when you investigate those three people around the car who have been turned to stone ... is totally solo. If you are with others when you trigger it, you will be 'teleported' to a 1:1 fight. It appears to be part of the tutorial.
After that one, the rest should allow you to fight them with whoever is present.
Big battles and silos should ask you when you trigger them if this is a solo or team encounter.