Joeytrom, that's exactly what I thought too! It's the only way Barnabas has to see if he's aged at all.
And yes, if we are to believe Bram Stoker, a vampire has to be invited in. Also, any guest of a vampire has to enter the vampire's house of his or her own free will.
Willie is wearing what must be his servant's livery--a white shirt with dark pants and vest. The poor guy actually flinches when he hears a knock at the front door, then panics and hides until he knows it's Vicki. This is the first she knows that he's working for Barnabas, and she praises the work he's done. He mutters his thanks, but keeps his eyes from her. When he realizes that it's almost sunset, he hustles the poor girl out the door. I don't think I'll ever understand you, she says. Don't try, he implores her, just leave NOW! Just as he's closing the door behind her, Barnabas appears, icily remarking, It was very considerate of you to get Miss Winters out of the house to keep her from harm. So _very_ considerate, Willie, he repeats, and I for one suspect Willie is not about to be rewarded for his consideration. He must suspect it too, because he’s shaking with terror.
Barnabas is from Europe, Vicki tells Maggie. That must be why he seems different, Maggie agrees.