If you create a group (e.g. "Test Group"), and assign it the URL of "~testgroup" (no space or hyphen), then when you post a node using the Automatic alias setting to that group, it will go to the subdomain of "test-group" rather than "testgroup" as expected.
Comments
Comment #1
redben commentedgo to
admin/build/path/subdomain
and check Restrict group names to letters and numbers
on admin/build/path/pathauto
Set group automated alias to [subdomain]