Only a 'Team Leader' (the person who created the team or was assigned the leader) can invite new users to join their team.
Some challenge organisers may restrict inviting - this will mean that only they can invite new users to your team.
In order to invite new users, you must go to the 'My Team' page on the web or app.
Make sure you are on the 'Team Members' tab on both.
At the bottom of the list of existing members there will be one or more 'Available spaces' depending on how many spaces you have left.
Click on 'Invite User' and follow the instructions on the screen
At the bottom of the team box, it will state how many users can join your team. If you reach this limit, you will be unable to invite anyone else without cancelling some of the existing pending invites.
Our tutorial video below covers creating a team and inviting people to join your team.