So you can't find a group for you. Why not create one - it's really easy. Select Groups --> Create a group... from the main menu.
On the iPhone or iPod Touch select the Groups tab from the bottom menu and then click the + button from the top right. Then select the 'Create a group' option.
You will need to choose a unique name for the group so you might have to try a few before you find a name no one else has picked.
Anybody who wants to join your group can, all they need is its name. Follow the instructions further up the page for joining a group.