Hi Geoff,
You have two options when posting to a group (or multiple groups) in Socialcast. The first option is to @mention those groups right in the share box (at this time you can only @mention those groups that you are a member of though that will change soon), or you can add groups to where it says "people or groups to notify." In both cases, start typing the name of the group and it will autocomplete (once the drop down appears, people appear at the top and groups towards the bottom). Sometimes you may need to pause for a second after you type the first letter or two of the group name for the drop down to appear.
The other option is to go to the group directly and post your message there. But, of course that does require an extra step so many users do prefer to just @mention the group name.
Hope that helps,
The VMware Socialcast Team