To ghost is to attend all or part of convention without a membership. Since sf cons nonprofits are run by volunteers, this is essentially stealing from other fans. At best, a ghost is a parasite. At worst, ghosts can help to destroy a convention as they did Boskone 24.
While nobody minds — much — if someone drops in for an hour or so in the evening, bear in mind that even if you don't sneak into any of the official convention functions, your friends or whatever you came to the con to see wouldn't be there if it weren't for the work and money put it into the con by other fans. Most cons offer one-day memberships if you aren't able to attend the whole event.