Have some sort of theme for the group.
How do I make a good group?
While a literal theme is nice, I believe he means a type of group. If there's something just called "my group," no one will join. (although there are some exceptions) If you make a group, people need a reason for joining. Like one for basketball, or baseball, etc. Make sure you check to see that there isn't alreay a group with your theme.
Once it's started, give people a reason to stay. Team matches, vote chess, tournaments, puzzles, and other stuff to do helps.
Remember: be innovative! If you want people to join, do something that no group has done before so that people would want to join.
I want to make a good group that everyone will join! Though it's hard, and my biggest group(that I made) has only 7 people!
Please help!