
Wait for the host to approve your connection.
Copy and paste that link into the bottom right of your parsec window
When the Host is ready, he will give you a link. Just, uh, don't poke around his mods too much, yeah? With Parsec, the program itself is all you need to join and play the host's version of XCOM. The host is streaming their game to you, so you do not need the game in your steam library, unlike the older Co-op mod that said not only did you both need the game, you needed mod parity with the host to make sure everything worked. The important note here is that: for you, the person joining the host, YOU DO NOT NEED A COPY OF XCOM. So while your friends may be able to hear the music, combat, and skittering of a chryssalid about to come around the corner and ruin your day, they won't hear your rookie's passive aggressive moaning, Tygan lament about Advent Burgers, or critical dialogue during cutscenes. This is fantastic, but parsec will also sometimes filter ANY spoken audio- unit lines, people speaking during cutscenes, etc. Mainly, parsec aggressively filters discord voice audio so your friends don't hear their own voices looped back at them as you broadcast your computer. *The Virtual Audio cable, while not NECESSARY to run Parsec, gets around a huge annoyance myself, and a few others, have found when hosting Parsec. They should now have mouse+keyboard control as well, seeing what you're seeing, and be ready to overthrow Advent together!. Click on their portrait again, and click on Mouse, and then Click on Keyboard to enable them to have mouse and Keyboard support. They'll be a little bubble/window at the bottom of the Parsec window when they attempt to connect, click on it, and click Accept. Copy the link to give to your friends (they will enter this link in the bottom right of their parsec window to join you). Go to the computers tab in parsec, and click Share Button underneath your computer (If you do not see anything, click the Reload Button in the top right). Now that you have XCOM running, you should see it on this list, put a check next to it to enable your friends to later see your game, and nothing else, when they join later. Tab to, or use the Parsec overlay, to go to the Parsec Setting> Approved Apps Tab. Start up XCOM 2 and/or WotC while having Parsec running. Download and install the Virtual Audio Cable fix here *.
Super easy once you've done it a few times, but the first time seems daunting. First time setup, and testing to make sure everyone can join, WILL take some time. Remember, you're going to be streaming to maybe up to other five people- you can't do that on a potato computer and a dial-up modem.
The best internet connection/computer out of your group of friends.