Select and register multiple people for a pre-selected event
That's a good question. This ought to get you started, if you get it all working please post your own answer with more details :)


Create a webform with 1 contact
Disable all fields except the "existing contact" field
Edit this field and set it to be "static", and set the default value to be populated from the group of your choice
Under the "event" settings for the webform, choose "register each participant separately" and set the event field to be "user select"
Edit this event field to limit the options to only the event of your choosing.
You should now have a working webform that shows the first member of your group and allows you to register them for the event.
If the above is working, you can now clone contact 1 as many times as needed to add the other members of the group.


Admittedly this method has a few drawbacks


Doesn't allow the end-user to select the group or event.
Contact name and event checkbox will be on separate lines (maybe some css could fix this).
There is a limit to the number of contacts you can have on a webform. Depending on the size of your groups, this may or may not be a problem.