Be visible. Practice where other people can see you, and always be prepared to tell people about that funny cup-and-ball toy. You already have a small crew, so get a regular meeting spot and time. And always be there every week. Even if it's only your local park, it's important for people to know that "the kendama players meet on Wednesday afternoons" (or whenever).
Since you already have a bit of a scene... Run an "event"! Advertise it in advance, with some relevant details. Tell your local skate shop, juggling store, local newspaper, local tv news show, get it known that "The Kendama Jam" is happening soon, that there will be a beginner's workshop, a stall selling kendamas, prizes to be won, etc.
That might sound like a bit of a scary prospect if you haven't done any of those things before. In which case, have a trial run, small scale, and make sure as many of your 15 regulars as possible show up. Give it a go, see how it feels, and I bet you find that everyone has a great time, and immediately starts asking "when's the next one?".
Wait... There are 15 players in your area? Why are only 2 of them on the Member Map? Tell them all to join! We said "be visible" earlier - that includes in online resources.