The achievement "Portal Jumper" requires you to "Join over 10 servers". It is widely believed that you need to join ten DIFFERENT people to get this achievement, but you don't.
The game is NOT tracking which servers you have been on, it is only tracking the LAST server you have SUCCESSFULLY connected to. If you join the same server 10 times in a row, it won't work. But we can trick the game into thinking the last server we joined was not the one we did!
You are going to need a friend or a kind internet stranger to help you, but only one, and only for about 10 minutes - this is far easier than finding 10 people or jumping through VPN hoops as others have been doing.
Step 1: Have your friend open up Besiege and host.
Step 2: Load up Besiege and join your friend by IP.
Step 3: Close Besiege and go to your Besiege installation folder.
Step 4: Open the Config.xml file in a text editor.
Step 5: Look for the line with LastConnectedAddress and an IP address.
Step 6: Change the IP address, even just adjusting the last number is enough.
Step 7: Save the file, re-launch Besiege.
Step 8: Fix the IP in the join field to be your friends again and join.
Step 8: Repeat steps 3 to 8 until achievement pops.
You can confirm the achievement is progressing by checking the Achievements window from the main menu of the game or the output_log.txt in the games installation folder, you'll see a line: [Achievement] Incremented Portal Jumper: 3/10