In one case that I'm working on at the moment, a shared inbox has a security group with a list of users. This security group has full access and send as access. 1 user out of the 15 listed people does not have the ability to book meetings on behalf of the mailbox. According to the user the other 14 users can.
I have seen with my own eyes her colleague's ability to book a skype meeting on behalf of this shared mailbox.
The error message is a Mismatch User Account The user accounts in Outlook and Skype for Business do not match. Please make sure you are signed in to both applications with the same user name and privileges.