Rob: "Bob, can I test out your crazy science experiement?" Bob: "Sure, Rob, be my guest" "Be my guest" means "Go right ahead" or "After you"

when you go to cafe or on the train, you want to share a table with strangers, you would like to ask for permission by "anybody sit here? " Then, he or she may answer you "no, please be my guest"

means "go ahead" "do it if you want to"

So, you want to ask questions here on iTalki? Be my guest.