By default everything you do in Confirm is private. When you are working on a team, however, it can be useful to allow others to create job requisitions, add candidates to existing requisitions, etc. To do this, you can invite someone to your group. You can also create different groups so that certain jobs are only visible by members of certain groups.
People can join more than one group, so feel free to be as specific as you want.
When being invited to join a group, if it is an already existing user, they will automatically be added to the group. This means that when they log in, they'll immediately see anything associated to the group, e.g. jobs created for the group.
If it is a new user, the person will receive an email inviting them to join. Once they accept, they'll immediately be added to the group.