Add to the list of available volunteers by campaign-group.

Campaign settings has an "Include Groups" option.

In the available volunteers region... provide a button/form... to offer a list of groups from the campaign which upon selection, will add those contacts to the available region.

Additionally... in the search form (linked from need)... provide a similar form.
... maybe, might be clutter, we already have a group field.

Should probably have a sanity check to not load 1,000 group members in a mailing list. Address this concern in another issue, please.