The invitation System have been a great addition to the game. It encourage players to work and try to find new players.
BUT!, Is really difficult to bring new players to the game because...
1º People that don't play games are not going to start playing.
2º People that play games, have their own tastes, and is really hard to succed into making them to download and try a game they are not into.
Who likes sports games is not going start playing shooters, who likes driving games is not going to try real time strategy games...
Games are like passports, Personal and non-transferable. I mean, everyone that like playing games, like to do it at his own pace, at an specific time of the day and with the games or their unique liking.
With co-op games like this, and every co-op game, It is really hard to coordinate 4 different persons agendas to be syncronized into playing the same game, at the same time.
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Me personaly, I have spammed in the whole Steam friends list, and real gamer friends, posted at gaming forums out there.
I have managed to succed with one real friend going to his house and almost forcing him to download and try the prologue ( ...He still playing his sports games), And haven't succeded into bringing other gamers from internet into the game.
Inside the group of people that is already playing Raiders of the Broken Planet is were you have better chances to use your invitations.
So, Why don't you try to enable invitations straight from the Matchmaking.?
Allow players that don't own the Episodes to join the matchmking of the premium missions while allowing the owners of the episodes to enable their invitations.
Introduce in the Matchmaking Menu information about if there are online players with invitations available so the players that don't have the missions can join.
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Is this going to affect Sales? I think it won't.
Is this going to breake the Economy System? Maybe, but i don't think so.
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The amount of invitations available is controlled at all times by MercurySteam, so if they detect abuses of the system they can correct them almost in real time.
At the begining, Free Players can have plenty of options to play for free because there are plenty of unused invitations.
But those run out really fast!. The average number of invitations obtained is 15 / week.
15 invitations are enough to invite 3 players to 1 episode of 4 missions = 12 invitations needed. And you can finish the episode in two hours of gameplay.
So, available invitations will never be enough to allow people to play constantly for free.