1. As a general rule you assume 1/2 lb meat (plus sides) for each person;
2. Don't stop your briskets halfway through the cook process. Finish them, rest them, wrap them, refrigerate (or freeze) them;
3. If you're refrigerating brisket, you should do so in paper or foil. To reheat them do not remove them from the paper or foil, just stick them in the oven at 250-300* while still wrapped;
4. Adding beer to any meat that you are reheating will absolutely change the flavor profile. Use beef stock for brisket, chicken stock or apple juice for pork. HOWEVER, if you're reheating an entire cut of meat then don't unwrap it. Keeping it wrapped will help retain the juices and keep the meat from drying out. If the meat is already cut/pulled then you add liquid to reheat; and
5. If you're doing a big party then chopping brisket/shredding pork in advance is an easy way to handle service. Read number 4 on how to keep the meat from drying out.