Hm. I can't seem to find anything that would be incredibly easy for everyone to do. There are a couple mods and chrome extentions that I'm seeing online that allow you to connect to the same i.p. address but that's about as close as I can get.

(Link 1, Link 2)

Edit: I actually just checked out the chrome extention (link 2), and it was super easy to set up. However, when I went to test it using my own i.p address, it didn't allow me to join properly, and the leaderboard still listed the game I was previously in. Maybe it would just need some testing, but I think that it will only be a mod or something along those lines that allows us to all play together. I'm going to keep looking, maybe someone created a game similar to agar.io but that was built to play with friends only.

Edit 2: Okay good lord. It looks like the game actually did have a function to play with friends, and was purposefully disabled by the creator. (See here) So I think any extention that is managed to be set up will ulitmately get shut down. So I am completely out of ideas. Maybe in the future they'll bring the feature back? I guess we'll see.

I went looking on GitHub for the sauce.

vram4/Ogar · GitHub

This is basically what you need to make a server. You'll need to do some portforwarding or use a different type of networking, but it works. Do read what's under it though, you need Node.js and the ws modules.
You can set up IP, amount of players, add bots and a lot more. It's really simple.
For portforwarding, the ports are :80 and :443. If the ports are in use, it's due to Skype so close that down.
It has a console so you can add rules when you want to. We might be able to get a server going if we ask Heiko nicely.

Hope this helped!

If you don't want to do portforwarding, you can use a VPS ( Virtual Private Server)

If someone would know this it would probably be the developers: agariogame.com/contact-us/ (link to contactform on their website).
Maybe they are willing to help you out or even set something up for you/us.