shy baby

C
My LO (almost 6 month) is the happiest baby you can imagine, like right out of a tv commercial. She smiles, squeals, babbles and laughs the whole day - until there's another person other than mommy or daddy around. It doesn't matter if she knows that person well or if it's a stranger, she just puts on her serious face and doesn't make a peep. It can be someone visiting or us going to a family bbq, it's like she just shuts down until we're alone, and out comes happy baby again. She lets other people hold and feed her, but no matter how hard they try, she never pops even a smile. 
People always make comments about how serious and quiet she is, and all I can say is she isn't when she's only with me and/or daddy.
Does this sound normal, is this just a temperament thing, or should I be concerned about this?
I just feel bad for our family and friends, they love her so much and try so hard, I know they'd love a smile in return 😕