Can I make an online multiplayer game using AppGameKit?

Yes, but currently only with other AppGameKit games.

I think Hishoo wants to make a game that is played ONLINE through the internet with players not necessarily being in the same room. Currently you can only make games using wifi with users on the same wifi network

The JoinNetwork command allows you to connect to any arbitrary IP:

Joins an AppGameKit network hosted by another app. The network is specified by IP address and port, alternatively you may connect by network name if you on the same LAN as the host. In the case of internet connections IP and port is the only way to connect as broadcasts will not leave the local area network.

Or does the command not work?

What is needed is a portal running on a server that can get and resend packets. I can knock up a test one if you all think it might be useful.

And I can Host it
If needed