You can view all the invitations that have been sent inviting people to join the system by going to the user index and choosing the View recent user invitations action. There will be one entry in the table for each group that the user was invited to and, if they are not yet a register user of the system, one entry for the system invitation.
Within a particular group you can view the invitations that have been sent inviting people to join that group by going to the member index and choosing the View recent member invitations action.
Both these pages show similar information with details about who has been invited, who invited them, when the invitation was made and the current status of the invitation, including where it currently is within workflow.
If a user has not received their invitation for some reason then you can resend it by selecting the dropdown menu next to their name and choosing the Resend invitation option. If they have not yet accepted and you want to cancel the invitation then from the same dropdown menu you can choose the Cancel invitation option.
For information about allowing invitations, see How do I allow users to invite other users to join?