Skype for Business: OK button greyed out when adding team call or delegate member

An issue that I see every now and again is the inability to add a user to a team call group, or as a delegate member within Skype for Business.

When you attempt to add the user, the OK button is either greyed out, or, when you click it the user is not added to the list.

Resolution

To solve this one, simply login to Skype for Business as the user that cannot be added. Once done, log out and log back in and add the user into your team call group, or as a delegate.