in your list of groups, you'll see all of your own repos and all repos where you're listed as a collaborator (On GitHub, go to repo -> Admin -> Collaborators). so people that are just watching or have forked the repo don't get added to the list.
in its current form, gitdash sets up a private mailing list for the committers, whether the repo is actually public or private.
One question I don't see answered is who are my collaborators? Is it everyone who is watching my repo or everyone who has committed to the repo?
Otherwise this looks like a really neat and useful service. UI looks quite nice