I would say be yourself.
What type of group is it?
There are groups for spamming, joking around and having a good time.
Then there are groups devoted to aspects of the game, like tiers or pokemon.
The former last longer, but require lots of effort from the owner, making friends with people and giving them something interesting to talk about or do.
You have to keep getting new members, because a lot of people are not active on smogon for more than 6 months or so.
The kind centered around pokemon or tiers fade out over time, but could keep going if you kleep getting new people and if you have some kind of tournaments or thingsto keep people coming back.