Great, you unsubscribed a contact!
Now you realised you make a mistake. Whoooooops!
So what do you need to do to reactivate these contacts you ask? Easy!
Go to Contacts > Unsubscribed Group.
Check all the contacts that you'd like to reinstate and then click on the Re-Activate link at the bottom of the table.
Boom. All done. These contacts will now be back into their original groups.