If someone shows up very early you are under no obligation to let them in.
I try to have the place clean and ready for guests 1/2 hour before the stated start of the party (most of the guests will be coming by public transit or through a lot of traffic, so gauging travel times can be hard). Then, I'm ready to answer the door in between setting out food, arranging vegetable trays, preparing punch and things like that.
If someone shows up unreasonably early, then you can just not answer the door - "I was in my bathrobe", or "Oh, I must have been in the shower" are perfectly reasonable responses if someone asks where you were.
I generally have things plotted out, and unless I know someone well and know that they will be useful, offers to help are more trouble than they are worth.