You can send invites a number of ways, including bulk invites via the People list. To send an invite to a single employee, go to the ‘People’ page, then click the 'Options' drop-down for the corresponding employee.
Simply click 'Invite' to send the invitation to the nominated email address.
When importing employees via the ‘Add People’ option, if you include employee emails in the employee details, you can select the option to send an invitation email to be sent to everyone on the list.
You can also send bulk invites or send select people invites via the People tab. Either select the individuals that you want to send the invites to or select the checkbox at the top next to ‘Name & Access’, click the ‘Bulk Actions’ drop down box and select ‘Send Invitation’