No not always, it may mean she just wants to sit and talk with you some more to get to know you better, never assume that anything a woman does is just because she "wants sex" .. make sure that this is *really* what she wants before you make a move or you could end up offending her.
She wants to welcome you into her space and see how you behave. So don't blow it by assuming you're getting laid and acting accordingly. She is likely attracted to you and will be most likely to pursue intimacy of some kind if she feels relaxed and safe with you. So the best advice is....accept the invite but be a gentleman, make her laugh, and tell her how lovely she looks. You'll be in her pants before you know it ;)