There are two ways to assign an assessor to a topic or article. NB: You can also view this related article about enrolling creators.
1 - Once a topic has been set-up (by an admin or creator), Admins can add or remove assessors on the Access tab of that topic/article (Admin/Knowledge/Topics and then click on a topic and view the Access tab for that topic - similarly for articles).
The first panel on the Access tab can be edited and assessors can be added or removed from the topic.
2 - When an Admin views a person (Admin/People/Users and then click on a person), they can use the Topics and Articles tabs to enrol that person as an assessor (or switch enrolments from explorer/creator to assessor, and vice versa).
Can a topic/article have more than one assessor? Yes.
Can a topic/article have no assessors? Yes. You only need to assign assessors when there is something to assess or review!
If I remove an assessor from a topic/article, what happens? Basically nothing. That person will lose the ability to assess the topic/article, but nothing else will be impacted.
Can someone be an assessor of more than one topic/article? Yes.
Can someone be both a creator and assessor of the same topic/article? No. A person can only be a creator OR an assessor of a particular topic/article. Creators also have assessor functions. Assessors are limited to only assessor functions (and cannot edit/create the content).
How can I see who are the assessor of a topic/article? Admins can go to the Access tab within a topic/article and the first panel will list all the assessors.