A protector is whoever "The Keeper of the Magic" is and protects the book with their two best friends (1965 - Present).
The Protectors don't choose when The Cookbook comes to them or when it leaves them.
Grandma Becky threw the book over The Falls after and Mama P and Gina Silvers cursed each other. This was seen in Episode 1 of Season 301: Just Add Time Travel.
Chuck Hankins (Charles Peizer) and Rose Peizer are the earliest protectors we have seen in the series.
In Episode 11 of Season 201, the girls (Hannah and Darbie) thought that Kelly Quinn had a problem because she had been cooking on her own nearly all the time and become too reliable with magic.
In Season 301 on the last episode Just Add Goodbye, we see that the cookbook leaves the girls and appears in a box where Zoe finds it, indicating that she is the new protector of the book.