I have the same issue. Cannot find a place where I can add a second or further contact or number to extend an active call to a group call - a simple and stable feature in the PC version of Skype.
If support/anyone can give us an indication of where we can find the feature or alternatively when it will become available that would be great.
In fact the Linux version on my Ubuntu PC can make group calls, but not iPad? This is not OK Skype/MS! - dont battle with Apple on this issue. You should deliver as good a product as possible even though it is on the IOS platform. That would be the best long term strategy for you, - in my humble opinion.