Besiege

Just finished this and thought I'd post to help others before the game leaves PC Game Pass.

Step by Step, this is how me and two others just unlocked it:

Prerequisites
You need at least 2 to boost. As mentioned, we did it as a trio and had no problems, but the in-game tracker went up when me and one other went between each others games. Therefore 2 people working together should be able to get this.

1. From the main menu, everyone selects the black hole - 'Multiverse: Multiplayer & Level Creator'.
2. Make sure Playfab is highlighted Red and not LAN.
3. Make sure Host is highlighted Red and not Join.
4. First player clicks the big red 'Host' button to the right.
5. Once loaded into game, you will need to use the Xbox overlay to invite your partner(s). Start by pressing Windows Key + G, this should bring up the Xbox Game Bar.
6. Make sure your partner(s) have been added as friends. Look for them in the social tab, right click their name and click Game, then select 'Invite to Game'.
7. Your partner(s) should receive a game invite, similar to what they would see on Xbox. To join the game they can either click this invite; press Windows Key + G to bring up their own Xbox Game Bar and either select 'Accept' in the notifications tab or alternatively, find the player who sent the invite in their friend list, right click on their name and click Game, then select 'Join Game'.
8. Once one of these has been completed, your partner(s) should load into the game.
9. Both players can then back out and then switch roles.
10. Restart at Step 4 again and run through everything again with the roles reversed. Once this is done, switch again. You can continue to switch round until both of you unlock the achievement.
11. To keep track of your progress, click the red house in the top right corner, then 'Achievements'. Scroll down to 'Portal Jumper' and see how many times out of 10 you have done. Remember, only the person joining will have their total go up, not the host.
12. Also note that you can't back out and rejoin the same server. Both players need to come out and create a new one for progress to go up.

A quick guide, but one that would have saved me a few hours of trial and error, as well as research online in trying to find a solution to join games. Hope that helps.

For those who wanted to complete this solo:

1. You have to create/use another user account in your PC.

2. Login to the game using the other user account and host a LAN game(you can also try PlayFab but it’s a long tedious process to share the PlayFab id across the users).

3. Now switch back to the account you want to unlock the achievement in.

4. Open command prompt or PowerShell and run “ipconfig” without quotes, note the ipv4 address

5. Open the game and join LAN and enter the ipv4 from previous step. This will count against the progress.

6. Disconnect from host and come back to join screen now enter 127.0.0.1 join the server this will increase the count.

7. Now repeat the above 2 steps until the achievement pops.

You can check the achievement progress in the home screen under “Achievements”

Update Jan 22nd, 20204: I've had two users who reported they had to attempt this method several times before it would work. If it doesn't work the first time, try repeatedly -- eventually it will work and you'll get achievement progress.

I've known at least 4-5 people who bugged out on this achievement, so I'm posting my guide on how to fix this broken achievement (or any, really) below:

Do you have any achievements not properly unlocking? Try this test to see if its this common problem:

1.) Go to the main menu of the game, then click on "achievements" (top right).
2.) Scroll through the list until you find the achievement that you believe should have unlocked but didn't (for multiplayer achievements, scroll to the very bottom, then check the "top" of the bottom of the list)

If you achievement is marked as 1/1 (or the bar is filled for cumulative achievements) OR if the bar is no longer moving on the "Portal Jumper" achievement like it should, then you have the achievement bug, and your achievement will not unlock naturally.

Fortunately, there is a method to fix this without having to completely destroy your game files and/or having to completely restart the game:

1.) Completely shut down Besiege
2.) Disconnect from the internet (i.e. shut off wi-fi and/or pull the ethernet cord)
3.) Start up Besiege. You'll get a series of error in a yellow box on the home screen. This is fine.
4.) Click on "achievements" (top right). Every achievement should read 0/1, 0/10, etc If so, you've successfully completed Step 2. Back out of the achievements menu
5.) Click on "mods" (top right, next to "achievements"). This will bring you into the mods menu.
6.) Reconnect to the internet. Wait 30 seconds.
7.) Click on the blue "Workshop" button at the top of the mods window. If clicking on this does nothing, your connection hasn't been re-established yet. Wait a few seconds and try again
8.) Once the Workshop button works, you should start seeing images load into the Workshop screen. Quit this menu and get back to the main menu.
9.) Click on "achievements" again (top right). Every achievement should still read 0/1, 0/10, etc If so, you've successfully completed the process.

At this point, so long as you don't quit and restart the game, Besiege will think you don't have any achievements, and will now unlock them properly. As well, since level progression doesn't need an internet connection to verify, you will be at the same point in the game as you were before doing this trick, so you can load straight into any broken levels.

Keep in mind progress on achievement will be lost once you quite this game, so make sure to do any broken progressive achievements (joining 10 servers, killing 1000 enemies) during this session.

I've verified this method on several accounts and on both multiplayer and single-player achievements.