Teams are created as either ‘open’ or ‘invitation only’ - you will automatically see both open and invitation only teams when you first join a challenge.
To join an invitation only team you will have to ask permission to join a team, otherwise, if the team is open, you can simply tap the join button and you'll be added to that team.
However, if you will also be given the choice of creating your own team - simply tap the 'Create Team' option where you can then add your chosen team name, send out invitations to prospective team members, amend your welcome message and set your team to open or closed.