BYOB's (Bring your own Bemani) are what I'd suggest. Although some members won't have their own bemani, people can still come and play games all night, do a few non-bemani related things as something else to go. You can all pitch in and get some food, or have them bring it - and tell them they can bring friends and just make it a fun thing to go and do.

Since you only have 8 players, a tournament wouldn't be as fun, and you can't get new people that interested by showing off with crazy 10 foot songs. Training sessions make the people showing up sound like they're morons. You could teach people how to play at a byob without any "formal training" whatevers and everyone will like it better.

But several bemani games to play, taking turns and hanging out - maybe a board game or two as well so that you guys can just hang out and not have it be about bemani 100% of the time.

Do things that you are calling "beman parties"

try to get members to bring friends that might be interested.

I'd suggest you guys compile your skills and money and work on getting a stepmania machine set up with coin slots that you can have at 25 or 50 cents a game somewhere in the facility.  If you can, use your own songs and steps, for legality purposes.  For the club, you guys can use it on freeplay with whatever songs you want on it.

People like GUitar Hero.  Try to use that to your advantage without turning it into a GH club.

You should do Amplitude/Frequency if you just want to use regular controllers.
Mini-tourneys are good.  Give small prizes, eg games and cash.
Don't forget teh food!

Don't get too nerdy.