As admin in the system you will have access to invite users to the system and it is often you will need to send more than one at a time.
The screenshot below along with the steps provided should give you a nice easy guide on how to invite your users to your system.
1. Go into your 'People' list on the left hand side.
2. Find the "select all" button to select everyone. Or pick and choose users to invite.
3. Once you click to include users you will see a blue bar appear across the bottom.
4. In the blue bar press 'Send login invite' to email all selected users with an invite.
(Users without an email address entered will not get an invite)
You can also send an invite to any user individually from this screen, you can tick one individual as shown above to invite. Below you can see how to also invite from this page.
Below is another technique for inviting users, this will be for one user at a time.
1. Go into People list on the left hand side.
2. Go into a user's profile by clicking on their name
3. Top right you will see the 3 dots and in turn see the 'Send login invite'