Some people also say "be my guest" in a sarcastic manner. For example, if someone just grabs your pen or book without permission, you can say: "Sure, just take it! Be my guest, I don't need it!" A non native speaker may not even realise this was being said sarcastically, as you would have to listen to the tone of voice and be able to read facial expressions.

There are two occasions. The first and the one that I believe you are inquiring about can be used as follows: 1) You use it as a way of saying "no problem" or "go ahead" when you answer a request. For instance: Mr X: "Would it be okay if I borrow your car for the weekend?" My Y: "Be my guest." It means,,, no problem, go ahead. If we think a little deeper into it... Be my guest literally means: "be a guest at my home" // "let me accommodate you at my home". But when it is used in this "no problem, go ahead" kind of way, what we are really saying is: "let me accommodate your request". It can, of course, be used a second way: 2) Just to say that somebody can stay at your home, as your guest. eg. Mr X: "I have nowhere to stay in London over the next week or so." My Y: "That's no problem, you can be my guest. Feel free to stay at my house". -- I'm quite certain you are referring to the first usage. I hope this helps you. Please just post a comment if you want any further explanation and I will be happy to accommodate you. Be my guest :)

be my guest = an invitation to the addressed to be your guest, to stay with you, to offer him or her food and drinks and host him or her.