I should also mention...it you want a new community (MH moderated), then you should go into the forum "MedHelp suggestions" and put forward your idea. Lots of new communities have been created that way if they make sense and don't overlap too much on another.
You can't actually create a community but you can create a group. The difference is that groups are self moderated as opposed to communities which are MedHelp moderated.
To do this, click on the top of your home page on forums. At the bottom of the drop down box you will see a tab for groups (or might be called user groups, I can't remember). Click on that. At the right side of the page, towards the top, you will see the words "create a group". Click on that, and it should walk you through it.
The best way to get people interested is to send personal messages to those you may want to have participate, along with the link to the new group.
Good luck to you!