There should be a recruiting system.
That will say, each player can recruit their friends by mail(max like 3/month) and if they pay for a month, the recruiter gets a free month himself.
This is kinda like the system Blizzard uses with World of Warcraft

This sounds like a good idea. The question is for me whether the people recruited will stay with the game in its current, non-finished state, and whether this wouldn't hurt GSW more than it'd help.

yea its nice but we should wait till 2.0 soon (tm)

Perhaps, yes.
But the game is lovely as it is <3 ^^
Too bad I can't afford my 1st month atm

It's not that expensive :)

And sadly, it's not likely that many people are willing to put up with the game in its current state (that is not the same as saying the game is not fun to play).