It looks like for the new Socialcast that you will (hopefully temporarily) have to select the group to post into by clicking ""Groups"" on the left then going to the group and posting there.
For the time being I have not found a way (I'm using Chrome on a MAC) to use the drop down ""Share With"" to share into a specific group.
Perhaps Sage is acquiring Socialcast as this release seems to be on about the same level as Sage's efforts....