First feel free to check out our article about Gaining Site Adoption to set yourself up for success.
Go to the Admin options (only site administrators can invite new users):
Select "Invite people" in either the left column under "People settings" or the link in the center "Administrator control panel:"
Type or paste in the email addresses and select an access level for all the folks you're inviting in (here are some considerations for site access level):
Optionally, you can select a team or multiple teams for them to automatically become a part of in addition to a personalized welcome message:
If a user accidentally deleted or missed the emailed invitation you sent, instead of re-inviting them under "Invite users," click "Manage invitations."
Look for the email address of the person who needs to be resent the invite, check it off, and click "Resend." You can also delete invitations from here as well.