Yes, you will be seated right next to strangers. They fill all seats at the table usually.
I have been to Biergarten many, many times and I'm not really a social person. Most of the time, the experience has been fine. The people next to us have been really polite and we've struck up a conversation about where we're from, where we might be staying, etc. A few times, we've had people who have pretended like there was not even another party at the same table as them. That was fine with me too. lol
I only had 1 kind of weird situation and that was when the restaurant was basically empty for lunch and they sat us with 1 other party. There were 3 of us and 2 of them. They sat next to each other, leaving us to sit across from them (they made us all sit at one side of the table because they said there would be another party joining us- but none ever did). They didn't say anything the whole time, to us or to each other. It was a little strange, but like I said, that was only once and I've eaten there upwards of 20 times probably.
I always find it is better to make sure that you are the first people to get to the table, so make sure you head in right away when your buzzer goes off and follow the CM closely to your table. I like to pick seats at the open end of the table (one end of the table is against a fence and 1 is going into the aisle), so I don't feel so blocked in by the other family. It is also easier to get up and go to the buffet.
I recommend going and enjoying yourself! It is a great restaurant and the show is a lot of fun!