Sheba, a couple of tunes which lend itself to wonderful group sings are:

Roll Old Chariots Along
Will the Circle Be Unbroken
The Last Shanty
Marching Inland
The Sailor's Prayer

Hope that helps. (The last three are Tom Lewis songs). You might want to check into a group called Artisan. Artisan's Web-site. They have new and traditional songs with lots of great harmonies.