When a user starts a discussion relating to a specific activity (either from My Activities or the Library), they will be able to view whether any experts have been allocated to the activity within the pop-up window.
They can then choose the Ask the Expert option when composing their post.
The experts will receive an email notification that someone has started a discussion and will detail which activity it relates to and that it is an Ask the Expert discussion. 
Clicking on the link in the email will take you straight to the discussion.
You can now type in a reply which can be viewed by the author and any other experts.
Click on Post reply.