How do conferences / journals select reviewers? When you submit a paper / manuscript to a peer-reviewed venue, the organizers usually forward your work to 2-3 other "experts in your field" to assess your work.

I'm curious how this process of selection works, i.e. how is this pool of reviewers put together? and how are the best people from this pool chosen once there is a paper to review?

Is there an official list that each venue maintains and to which people can sign on if they want to be reviewers? (If so, I've never seen any "calls for reviewers" so how do they get people?)

Do organizers cold-call people/professors and ask them if they were willing to review a paper? If so, how are these people selected? [I realize that this might differ considerably, so I'm happy with anecdotal evidence, too]
In the journals I help edit (all in the medical fields), there are five ways we find reviewers for our manuscripts:


They have published with us before
They were nominated by the authors, editors, other reviewers or peers, or members of our editorial board
Our associate editors approached them through a scan of authors of papers on the same topic
They have volunteered to be placed on our list of reviewers
They work for us as consultants and are paid on a per-review basis (this is mostly relevant when we need specialist expertise such as in statistics or economics)


Once you are found, you are placed on a pool from which we can make selections on the basis of affiliation, expertise, seniority, etc.

As you can imagine, this is a rather valuable resource. We guard this list jealously and do not share this with other journals. 

In my experience the response rate is typically 33%. That is, if we require five reviews, we will invite fifteen people. Thus, the pool needs to be deep in order to receive the requisite number of reviews for our purposes.