I made a mod for Slitherio. It requires Grease-/Tamper-/Violentmonkey to run, and it can be installed from Download button. (Its named that, as the fps display was the first feature in it). The code is completely human-readable from the file.
It does not replace the original Slitherio script, and it does not require you to block the original Slitherio script, its a very unobtrusive overlay to the original script.
To join your friends, have them give you their server ip and port (it will be shown by the overlay), insert it into the IP-Address:Port field (as x.x.x.x:yyy), click Go!, then play game. You will now be in the same server. To find each other, you can use the coordinates shown in the overlay. the higher x is, the further to the right. The higher Y is, the further down. You’ll figure it out.
I did not test the script with Opera yet, just Firefox and Chrome. Gimme heads up for bugfixes. If its popular, I’ll add more features for sure :).
Thanks to @MetalMonta
Make sure you disable any other slither.io extension and scripts first!
How to install user scripts?
Firefox and related browsers: Greasemonkey.
Google Chrome, Chromium, and related browsers: Tampermonkey.
Opera (version 15 and later): Tampermonkey or Violentmonkey.
Opera version 12 and earlier supports user scripts natively. Violentmonkey provides a friendlier UI and better compatibility.