1.) Battle.net --> There's always a bot hosting by [ENT], you can see it on all server, but most people are on "US East and US West". You can try to join "Clan FS" if you see Clan Shaman or Chief there, although it's not very active. The game name looks like " [ENT] Fortress Survival! #(number) ", and you can window out from gaming screen and go to explorer and type "entgaming.net" --> go to "game list" --> scroll down to see the game name in alphabetic order to see the game number.
Also, there's a bot hosting by [EU], but you will see more players play in [ENT] rooms.
2.) Garena --> because many classic games are played in old patch 1.26 and no one would update those maps, so if you play other games on garena, you will need to google how to upgrade and downgrade patches between 1.26 and 1.27. Second difficulty, people play in different countries, you will need to use virtual network connection for local area connection to work. (Garena has closed LOC of many countries). Some people play in Philippines area.
3.) Eurobattle.net. . . you'll need to ask someone else for it. I have never tried it.
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Social media:
We have Garena group, Skype, Steam, (and probably something else that I forgot or don't know). I would say that Skype works best. Just search for the Skype connection/invitation thread somewhere in this forum, or leave message at bottom of website like... your skype account name (if duplicate, how to distinguish such as profile picture...etc) and ask someone to add you to the group.