Should you wish to add another party to your actual mortgage, you would need to refinance the loan. There are costs involved with that, those being the costs that come with the typical refinance of loan.
But should you hope to add another party to the Title ... which refers to ownership ... you need to contact an attorney to accomplish that. Attorneys fees and title fees would be involved in this act. Two different outcomes are served by these differing actions, so I would suggest that you be completely clear and comfortable with whatever action you take ... and also well-educated and versed before doing either.
If I can help clarify anything or answer more questions, please feel free to contact me either here at Trulia or via my website.
Best of luck to you ...