It would appear to be possible in Skype although I'm guessing participants would have to answer at the start of the call.
Click the “Conversation” menu at the top of the Skype window and select “Add People.” The Add People window appears.
Select contacts by clicking their names and clicking the “Select” button.
Click the ‘Add” button” to add your selected contacts.
Click the “Call Group” button to initiate a voice call.