They like you and clearly want to make sure you sign up with them. Be professional, and prepared as you would be for a fit interview (it may not be an interview, but you are already building your reputation). 

But it will likely be just a session to convince you to join BCG, or possibly to know if they need to adjust something to your specific needs (starting date, moving expenses, etc.). 

Bring questions (once again, don't be naïve, treat it as an interview, but you may still have legitimate questions that may help you on your decision or simply your planning for the next few months).

Congratulations!!

Hi!

The call is about understanding whether you'll join the firm and why you should join BCG, specific requirements, etc.

Congrats!

Anto

Hi Hannah,

Congrats on the offer!

Unfortunately you won't be getting a “sell weekend” because of the pandemic, so this is their way of trying to make sure you sign.

They want you, so all they're trying to do is make sure you're happy and excited to join the company!