I have been lining up for U2 for what seems like half my life. I have seen them in venues all over North America, though never in this area (just moved here). In every case that I have seen, it's more or less the same. The first few people in line start a running list of names that correspond to numbers that get inked on your hand. While you can try to negotiate with your line mates to have the numbers set aside for your companions, it's really up to the self-policing line to see if they'll allow it. I have literally seen a fight break out when someone tried to do as you are suggesting and new people showed up shortly before the gates opened. Granted that was in LA, but people in this region seem far more uptight and nasty than out West. I have to say that nerves can be a little frayed for those of us who have been in line for 20+ hours and taken our vacation time from work to do so. It just doesn't seem right that other people who haven't put in the time will cut ahead of you with no effort--not very respectful, you know?
As far as the lawn chairs go, that's fine. Just know that you'll have to store them back in your vehicle (or wherever) 1-2 hours before the gates open, or be okay with trashing them before going in.
hope this helps