This is quite a convoluted process I'm afraid!
Can I just clarify one point before I explain how it works - when you say that your 16 iPads are under one user - to you mean Apple ID or assigned to a user in User and Location in Jamf?
The book process works as follows
Finally, the iPad needs to be signed in with the apple ID that you assign the licence / invitation to in Jamf.
Hope this points you in the right direction.