A team is a group of people working with Q. to manage their projects. Usually it will comprise of the people that work together with you at the same company.
If you register for Q., you will be asked to create a team within the registration process. All you need to do is to enter your colleague's email addresses and names.
You can also invite new team members at any time later via the main menu.
Q. wants people to collaborate. Thus it is no problem be part of multiple teams and switch between them. To join a team that you haven't created, you will have to be invited.