Adding new users to Adobe Sign who already have Adobe accounts

I've just set up Adobe Sign solutions for enterprise for our business. I'm now starting to add users. The problem is that a number of our directors and managers already have an Adobe account. So when I add them as a user in Adobe Sign they end up with two different accounts. This creates a lot of confusion!

Is there any way of adding a user's exisiting Adobe account to our Adobe Sign? Or is there a handy user guide for switching between accounts?

Thanks!