Skype for Mac OS X
Conference calls
Up to five people can participate in a conference call with Skype. Starting a conference call is simple, just choose “Start Conference Call…” under the “Call” menu. Next, select the users you’d like to invite to your conference by selecting their names and clicking “Add to conference”. You can select more than one user at a time by using the shift or command key. To begin your conference call, click “Start”.
If you are already in an active call and would like to add an additional participant to a conference, you can select the person you want to add to the conference from your contact list, control-click or click the drop down menu button and choose "Add to conference".
Keep in mind that the host of the conference (the person who started the conference) is the only person who can add new participants. In addition, because Skype conferences are peer-to-peer, it's important that the conference host has a good internet connection. If you are planning to conference call with many participants you should choose the person with the best internet connection to host the conference.