elzie, the doors are open during normal business hours. And as you can imagine, whomever happens to be there at the time will pretty much drop everything to make you feel welcome (or coffee - thanks, Mary).
But if you want to make sure you will have a chance to spend a little time with anyone in particular (long list of possible names here), then you should just call them in advance and figure out the best date, because their schedules vary quite a bit, and sometimes they do actually book visits/tours, or someone may be out of town, and so forth.
I've been there three or four times now, and I always call or email in advance to confirm a good day and time.
(Message edited by bob on August 26, 2005)