Can I play with a friend on the same IP as me?

It is possible, but slightly complicated:

1. Refresh the page and stay in lobby.
2. Choose FFA or TEAM
3. Press F12 (for google chrome)
4. Click on "Console"
5. Find "Connecting to ws://xxx.xxx.xxx.xxx:xxx" (x's are ip)
6. Give that IP to your friend, who does the same, except he takes this IP and writes to the console: connect("ws://xxx.xxx.xxx.xxx:xxx") (replace x's with numbers)
7. Check if you're on the same server by matching Leaderboard player names.
8. If you're not, both write connect to the console. (it happens sometimes)

They've implemented a mechanism which allows to connect to the same server by sharing a URL. Check it out on the page: enter your name, click on the game mode, select Party, on the right side click "create" and copy & send the URL to your friends.

Enjoy!