This is quite simple to do:
1) Head over to the "Lists" menu and hit "Forms" of the mailing list the contact unsubscribed from.
2) You'll be taken to the forms page. Locate the list's default form (it's usually the first one), click its options button to the right and choose "Link".
3) E-goi will pop up the form's URL. Copy it and paste it in your browser bar.
4) You'll see the list's sign-up form. Now just enter the contact's email address and submit it! E-goi will then send a confirmation email to the contact (this is necessary to prevent unwanted resubscriptions). As soon as they click the email's confirmation link, their status in your list will switch back to active :)