Right now, steam user who refer 5/5 other steam users get a Steam Knight Mask.
So, people just invite those 5 people and do not bother inviting more people after that.
You realize that if you added more things than this, you'd get more players? Of course, not ALL of them would be active but neither are people who try out the game without being invited, they're not guranteed to stay for years to come.
I think we should have at 10 total invites, a Steam Knight costume armour, as well pumping out steam like the mask. I have no clue how it would look, I'm no designer.