Go to Settings > Account
Enter the emails of the people you want to invite. You can invite more than one person at once. Enter an email and then press the '+' next to it to add more. You can also just paste a list of emails.
Choose a role. The role decides what the people you're inviting can do. You can learn more about access control here.
Invite them. Just click the Invite button.
Next, they will get emails to create their account. Once they do that, you will see their status updated too.