Something to merit Azarch's words. While it is okay for the people who build/created the phases entire idea to be a few friends. It is advisable that anyone who plans on hosting a public phase be sure to contribute to the spread of RP on the server. Not simply posting the ad for the phase for people to join and then immediately going off elsewhere with friends or guildies to do whatever it is they want.
A great public phase tactic, I feel. Would be to invite others, whether through IC dialogue or a chain of events happening in the phase that could be presented through phase announce or other mediums. Something that would spark other characters to react and perhaps even check out what's happening. David's last post is a problem I've seen too many times. Where I've entered a phase, expecting others to be there, but left with a really, beautifully built phase. But the people who host it are often elsewhere with the group they choose to be with, without any visual queues of what's going on or even where they might be. While I get that can be sometimes hard, given the nature of some phases or activities within them. It is always a good idea to levy your options.
Do you want to host a phase to the public so that you can gather more for the phases' story, or do you want a more open ended place where ideas from all different types of people can be explored. Where a single factor could possibly change the outcome of what direction you're going in your phase. It's a tactic I employ in my time volunteering for tabletop RPGs at my local library. I don't /always/ have a group of set people, and I don't /always/ have something in my head that will go one way. Every new face is a welcomed addition to what could be the story.