Inviting Last Minute Guest

We have planned a wedding for a year now, stress has just arisien while struggling with the guest list. Here`s the problem we have a large family and limited number of seats. We invited all the closest family and a few friends. However, we have had three replies of family that are unable to attend. So I want to invite a friend from work but am not sure how to do it now that invitations are out and he knows he didn`t recieve an invitation along with the others. Should I forget about asking the friend or should I explain the situation and invite my firend? Any advice will be greatly appreciated.