All about the friendship and the bonds you make in the guild. Recruit team players. People willing to stand by you in your darkest hours. Do what's best to make those around you happy. If people are happy, they will want to stay/join. Try and give your friends and guild as much undivided attention as possible. People really appreciate that. You'll know you're a successful guild when the times you've asked people to join isn't as nearly as many as the times people have asked you to join your guild.

Oh, and be very weary on tag rules. No one has any right to dictate what others can and can't do. So don't force super strict tag rules on the people if they don't like them. It's more fun when people want to be on tag instead of being a dog on a leash. If someone is unloyal or inactive, get rid of them. It's not so much of how many people you can get in. More so, how many of those people can you get to stay in.