To join a team you need to go to the fifth icon in the bottom bar of the game. There, you will find different ways to join one.
Once you enter the section, you will see a search engine in which you can enter the name or code of the team you want to join.
You will also see the list "Teams looking for players", in which you can select the one you want. There you will see the information of each team, their name and number of members.
Keep in mind that there is a limit of 50 members per team, so you can only join those who have free spots.
On the other hand, by clicking on the "Create team" button that you will find below the list, you can start your own.