i suppose it depends on what size wheels as well?
Will people be able to sit in the back with it on lowered springs? as my car sits fairly low with some of my friends in the back lol 
no - the size of the wheels doesn't make a difference as you should reduce the tyre profile as you increase the rim size so that the total radius remains the same (otherwise your speedo will be out and the wheels may rub). For 17" rims you should use 225/45/17 profile tyres and for 18" use 225/40/18. This will give you almost exactly the same radius as your original 195/65/15s.
As for people in the back - mines lowered 40mm on Eibachs with stock shocks and I've had 4 people in the car and two heavy suitcases in the boot - no probs (in fact it looks sweet because it gets lower - unfortunately went a bit slow tho'

).
Hope this helps!