Hi there,
We have been twice - once as a group of 2 and once as a group of 4, and have NEVER booked. There are a few tricks though (well, my personal opinion anyway)
Get there early - and I mean early, there are a few non reserved seats. You can then move between the tents to see which are good. You want to be firmly perched somewhere by lunchtime though!! Some tables are only reserved from about 230 or even 6 so you can look at sitting at those until the groups come along. We found both times by the 2nd day, we wanted to leave by mid afternoon for a bit of a "breather", and to check out the actual beer houses in the cities anyway.
Yes you do need to be seated to get served. There are people that go around with trays filled with pickles, sausages, and booths inside the tents that sell pretzels etc - so you wont starve!
Tip the waitresses if you want them to come back to your table - they are run off their feet!
Be friendly! A small group like yours can be accommodated on a table and if you ask nicely, most people will allow you to sit there - especially as group sizes fluctuate throughout the evening. And even if you cant get on a seat, am sure they will let at least one of you sit down to get served drinks!
Be prepared to line up if you go in the evenings, especially weekends.
If you really want to reserve a table, go on a website like gumtree to see if you can get a group together.
Its messy, but its great fun! Hope you enjoy it!!