Just last month I considered adding an AU, and since my annual fee is in February I had the exact same question as OP. I asked Chase about this using their secure messaging. Their answer:
I'll be happy to address your follow-up inquiry about the
annual fee for each Authorized User.
CaptainMiles, the first $75.00 annual fee will automatically be
applied within one to two billing cycles once you add an
Authorized User. Going forward, the $75 annual fee will
be applied along with your $450 annual which is February 1
(Anniversary Date) of each year.
We appreciate your business and thank you for choosing
Chase.
Thank you,
Navin
Chase Email Servicing
1-800-493-3319
AOC: 6530