Besides the search, swimmers can also be added throughout the website, anywhere there is a list of swimmers, there will be a green button with a '+' sign in the middle, like this: Examples are the favorites page, teams page when a team is selected, and anytime you are viewing a ranked list of swimmers in an event or in IMX/IMR. Each swimmers' row will have the symbol. To add, just click on the symbol.
To remove a swimmer from your compare, click the blue button with the '-' sign. Once you are done adding swimmers to compare, navigate back to this page.
Example from the search page:
Example from the top swimmers in the 200 Freestyle: