Yes, you can join friends within X-plane 11
Google, "Whats my ip address" note it, get your friends to do the same, share. On the Settings/Network Tab, upper right, input your friends ip addresses adding a line each. Get them to do the same.
You may all need to open UDP port forwarding on each router/firewall (49000-49010) - check youtube for your models, if you dont know how.
Ensure you all disconnect xSquawkbox and iVap using PluginAdmin (Upper Left Dropdown Menu) simply untick.
LR need to invest a little work as currently multiplayer doesnt send livery data.
Hope this helps