Can you join via the lobby? If the host has online visibility set to invite only, they would have to increase it to friends only or public.
Does it make a difference if the game is already running?
Can you join other people, via invite or lobby?
Have you tried hosting, and inviting your friend to a game?
If your friend starts a new multiplayer lobby and invites you, can you join then? If so, can the host then load the latest save from your current playthrough?
Have you tried hosting/joining an Arena match?
Try going into Arena mode, create an arena, open the Connectivity menu and check the settings, then exit to the main menu and try joining in the lobby or accepting the invite. Alternately, create a multiplayer game lobby, exit and try joining.
If you are using any mods, try testing in a new profile without any mods enabled.
Try verifying local files: in the Steam library, right click on the game and select Properties, switch to the Local Files tab and then click on the 'Verify Integrity of Game Files...' button.
If you use Discord, those invites may be an option.
Did you try joining by Direct Connection, with a Server ID (which the host can get in the Connectivity Menu after starting a new game or loading a save) or IP address?
LAN may be an option, but since you are not on the same local network, it would require VPN software, such as Hamachi or Tunngle.
Try resetting your network adapters:
- click Start (or Search in Win 10), type cmd into the search box, right click Command Prompt and select run as administrator.
- type the following command and hit Enter
netsh winsock reset
- when that is done, there should be a message 'Winsock reset completed successfully'
- type exit and hit Enter, then reboot your computer.
For more details, see:
If you can not join other people, either, try starting the game, switch to windowed mode and lower the game resolution (and maybe check the option to mute the sound volume when the game doesn't have focus), start a multiplayer game, hit Esc, open the Connectivity Menu, optionally disable online multiplayer (if it is enabled) and make sure LAN Connections is enabled (or that can be done in the options before starting a new game).
Next, start the game again from the 'EoCApp.exe' program file (doesn't matter if the first instance is started through Steam or the executable), select Join Online Game in the Campaign section, switch to the LAN tab, rather than Online, and try joining.
If that doesn't help, please email firstname.lastname@example.org with a dxdiag report (WinKey-R, type in dxdiag and hit enter, then when it finishes loading click on the 'Save All Information...' button and save the report somewhere handy), or reply/PM with a Dropbox or Google drive link, etc.
Also check the '..\SteamApps\common\Divinity Original Sin 2\DefEd\bin' folder for the gold.log and network.log files, and if there are any CrashDump files being created, you can zip a few together to include.