Skype for business works differently to normal Skype, normally you can call a regular Skype user directly. However when multiple callers are involved, it isn't as simple as just adding them to the call.
Skype for Business, when used externally, is better used as a conference call system similar to GoToMeeting.
Instead of calling, you must use the MeetNow function then invite attendees via email or instant messenger.
Here is how to do it:
5. The call should now start. At the bottom right of the screen, you have a More Options button, click this.
Now you will have to select Meeting Entry Info
This will provide a link that can be sent to guests and externals which will allow them to join your call;
6. Copy this link and send this to your delegates via email or instant messenger.
They will then be taken through a few steps to join the session as a guest.