Ok lets start :).
Firstly, There is a Playtomic plugin, which will be very usefull for storing game stats, but it also has online leaderboards!!!! :D,
The actual multiplayer plugin is not out yet, but its being worked on by a amazing user!!
- you can follow the tread here: http://www.scirra.com/forum/multiplayer-demo-in-c2_topic47724.html
If you wish to make your own plugin then here is the documentation/ template for it: http://www.scirra.com/manual/15/sdk
note here is a link with a list of construct2 plugins.