"Guide" by Pvic
Can't really call this a guide but hey... this will probably help someone.
So, basically just look at the source code for Sonic &, my joke online game and download the GameMaker Server extension (download links in the download section)
The online code you will need from Sonic & are in
"obj_login"
"pGlobalPlayer"
"pOtherplayer",
something simillar to that.
Scripts I found useful and that you will probably be using the most:
gms_other_get()
gms_self_name()
gms_chat()
gms_chat_toggle()
gms_info_ping()
gms_self_set()
gms_info_isconnected()
PS: if you implement this in any engine, make sure there is no deactivation. In GMate deactivation is in obj_pause_screen, obj_title_card and obj_destroyer. You will have to mess around and remove the deactivation in those codes.