In the contacts tab, select the contacts you wish to invite by clicking their name, or select all contacts by clicking on the toggle all button (red arrow is pointing at it below)
Once you have the selection you want, click the envelope icon to invite contacts
You will then be prompted with an invite contacts pop up to confirm the contacts you wish to invite, once confirmed click 'Invite Contacts'.
Note: You can send reminders to any contacts that have been invited, but not yet connected. If you don't want to send reminders, simply un-check the blue tick option by clicking on it.
You have now sent out your contact invitations. You can check the status of the invitation within the 'Parent Accounts' section. It will display "Connected" when the contact has created their account.