I am currently in two Guilds. One is moderate sized. The other though we have so many people, (over 350 active accounts) that we recently started telling current members they can't invite alts because of all the new people we get. On average I would say we have about 4 new recruits every week or so.
RP is a major factor. Plus conquest groups. Heroics. Holding RP events every week. Doing OPS. All these things make for happy members, and keep our numbers very high.
What do you offer your Guild?