On the People tab, click the pink + sign at the top right.
In the Who are you inviting to your-team-name? dialog, click the appropriate Invite button (for either Members or Guests).
In the Invite people to your-team-name dialog, enter the invitees’ email addresses, and click the Next button.
In the Invite person to the your-team-name team dialog, verify that the info is accurate and then click the Invite button.
To let people join your team without requiring an invitation, you can set up Open Enrollment. With Open Enrollment enabled, anyone can sign in to the team by using an approved email address—i.e., an email address from an email domain that you've approved.