Click "Invite People" on the right hand side just below the members list. Or click the "Group Settings" button just above the members list, then click invite on the bottom left. 

Here you have two options. A shareable link that can be copied and shared through any platform, and a private invite by email.  

The link can be copied and send in any shape and form, and anyone with this link will be able to join your community. If this link has reached people by accident or that you would not like to include. You can reset this link by clicking "Reset link" and the previous one will no longer be functional. 

To send a private email invite, type or paste the desired recipients email(s) into the text box labeled "Type Email Addresses". In the next section below you can personalize a message that will be sent along with your invite. Then click "Send invite". Once the link has been clicked, it immediately becomes invalid.  

You may also view your Invites that have yet to receive a response under "Pending invites", and reset all of them with the "Reset all" button (this rescinds the invite).