mental shale
As Host
After clicking the "press any button" a prompt will appear "You will be able to play online with a friend if you sign in. Would you like to sing in to use online features?"
Click accept (both or all have to click it.
-Choose main story or family trials
-Choose your save files
Press ctrl and then click "Invite"
-It'll show your friend's name, click their name and the invitation will be sent
Invited
-Press "Tab and Shift" to bring out the steam UI
-Look for the "Two people" Icon and click it
-Then the list of friend list will appear, double click the name of the friend and the message UI will appear.
From there you will see the invitation
-Click the accept button.
If i'm not mistaken you will be transported to the host game file.
As the host, the game will proceed how it usually is when you play single player, the invited will have to wait at the lobby.
The invited can't do any upgrade other than adding character skill points or probably help host progress with the dialogue of the characters or story at the base.
The host is the only one that can do the upgrades around the base.
Warning
Any skill points added by the invitee on the character might affect the host's character skill allocation so always communicate with the host unless you plan to only play co-op and with the same character.
After the session
The invited could choose to copy the online co-op progress by replacing a save slot or existing save file.
To do that, go back to "Save Management" section
-Click on the the icon beside the save file. An interface will appear
-Click on "Copy online save" and a warning will appear.
-Click accept if you agree and there you have it. Now you will have your host's save file in your own slot.
Everything that the host invested into the character and the base will be in it. Including the amount of Morv.
If you want to progress any characters with a different build then communicate with your friends.
And that is it, have fun and Have a good day. 😄