You tell someone to be your guest if you approve of their using something that is yours. This expression is similar to "Sure, go ahead," or "Sure. Help yourself." Tell someone, "Be my guest," when they ask to borrow your laptop or use your restroom or change the T.V. channel, etc. ... and you are completely fine with it.
A: I forgot to bring my dictionary. Can I use yours?
B: Sure. Be my guest.
A: Thanks. I just need to look up a couple of words.
B: No problem. Oh, is that your highlighter?
A: Yeah. Do you want to borrow it?
B: Can I?
A: Of course. Be my guest.
look up - find information as in a dictionary or a phone book
Alright folks, the next time you want to say it's ok for someone to borrow something of yours, just say, "Be my guest."
Have a good weekend.