Depends upon the neighborhood. My mom lives in Forest Hill, and just had 30 people over for Christmas dinner. They were all able to find parking within a block. We are also walking distance to public transportation at West Portal, so a few took public transportation. Some other neighborhoods are a pain to find parking. We used to live in North Beach and that was one of the reasons we moved out to Forest Hill. Fortunate thing, San Francisco has decent public transportation. My mother hasn't driven at all the past 40 years she has lived in San Francisco. Just a note... you can get ticketed for parking your car in your driveway if you are blocking all the sidewalk that pedestrians are supposed to walk on. Happened to a friend of mine when they came to visit me in college... didn't leave enough room for pedestrians to pass...
If your house is on a street taht requires permits to park for longer than a few hours (the street signs will say), you can purchase a short-term permit for guests. Not so useful for a party, but if you have someone visiting for a week it is very helpful.
You should really inform your guests that they should come by taxi if they live in the city. That way they do not have to worry about parking or driving home afterwards if there is drinking involved. If they live outside the city your options are limited. Just inform your guests that they will need to be patient
They'll park in your driveway, even if that crosses over the sidewalk. All over the city, you'll notice that people park in driveways, or on the street blocking driveways. A little known fact, however, is that it is illegal to park blocking a driveway, even if that driveway is your own. That said, given how difficult parking is in many parts of the city, sometimes you have to do it!
That really depends on where you buy a home. In some neighborhoods, they can park on the street but in others that is not possible. For example, in the marina or in Pac Heights your guest may drive around endlessly. In Russian Hill you're likely to have the same problem. Time of day, of course, impacts parking.