You don't need to come with a team to CodeDay. In fact, most people don't come with a team!
At the beginning of the event, everyone has the chance to pitch ideas for apps or games they want to make. (See "What can I make at CodeDay?") You can find an idea to work on then.
Teams can be made up of anyone you'd like, they don't need to attend your same school, be in your same grade, etc. Even volunteers can be on your team, if you can convince them!
What if I want to come with a pre-formed team?
That's totally ok, too!
Remember that, if you've done any work in advance, you need to disclose that to the judges. Doing work in advance might disqualify you for the "Best in Show" and "Best in Class" prizes. (See "What can I do before CodeDay?")
What if I decide not to work with a team at all?
Also totally ok; many students work by themselves. We do find that students have more fun when participating as a team, however.
Judges will do their best to take the size and experience of each team into account when deciding on prizes.