When viewing the administration area of an Activity or Pathway (see find content in Find Learning or Content Manager if you're unsure how to do this), click on the Discussions tab on the left-hand side.
Click in the Search and add users... field under the Experts heading.
Start typing the name of the first Expert you want to assign - the LMS will suggest possible matches based on what you type.
Select the correct name of your Expert.
Repeat the process until you have selected each of your Experts - they will be listed under the Users heading.
Click on the Save button at the bottom of the page.