My friend just did this, and it went great! I get to be her helper.

The way she started is by finding all the HSers she could and getting their emails, and starting an online email loop. Then she made guidelines for posting, and invited everyone to a planning meeting and a picnic.
At the planning meeting, we all had ideas like a field trip here, or a craft night at so-and-so's house, or calling the school to get an open gym time once a week, etc, etc.
Then whenever something is going on, she posts it on the list and we can go. Other people can post stuff too, about local events or really whatever. It has been maybe three months or so... and it's going great.
I think the best way to find other HSers is to hang up signs at places they go, like the library. You could mail flyers to churches, LLL meetings, 4-H, health food stores, and ask them to hang them up. Put a free anouncement in the community paper or free community radio announcents... Before you know it, word of mouth will have it's own momentum. You could also contact HS groups in the area if you can find any, and ask them to put your info on their newsletter or what they can do to help you. (I mean big groups, maybe like umbrella schools, or legal groups, or religious HS groups, or whatever.) Don't forget MDC Finding Your Tribe.

Good luck. I love our local group and it was really worth it.