OGMP is the OverGrowth MultiPlayer mod, an unoffical addition to the upcoming game Overgrowth by Wolfire Games.
To install the mod you simply have to copy the ogmp folder to the Overgrowth mods folder (e.g., My Documents\Wolfire\Overgrowth\Data\Mods, ~/.local/share/Overgrowth/Data/Mods, ~/Library/Application Support/Overgrowth/Data/Mods).
To connect to a server you first have to load a level. Once the level is loaded you can press P to open the connection window. You can either connect to a public server like ogmp.net or you can host your own server with ogmp_server.
Once you are connected you can press P to see who is online. You can also use K to save a position and L to teleport to that position. This is very handy for parkour levels. You can chat with other players that are on the same level as you by pressing enter.
This mod is still early alpha and buggy. If you encounter a bug it would be nice to send us information about it via the issue tracker of Github. Please try to avoid duplicates though.