I have found that the most successful groups that I help are groups that already have something in common, which are called "affinity groups." For example, family reunion groups, church groups, and scrapbooking groups are more successful in recruiting cruisers because there is a shared interest that draws people in. It's harder to get people to become part of a group where the only thing you have in common is that you're all sailing on the same ship at the same time.
Personal contacts can work well, too. Talking to people you know about joining you on a cruise can work, because YOU are what they all have in common. I find the least successful group leaders are the ones who just set up a cruise date and hope that people will join them on that particular cruise.
As for reaching a particular group size, I'm not sure what size you have in mind. There is usually a "perk" for the group leader for every 8 cabins s/he can fill (based on minimum 2 per cabin) on any sailing.