It was her boyfriend and a friend of his. It was mentioned on the comic but not on the TV show (as far as I remember).
In the early stages of the outbreak, Michonne runs on foot to her
house. This attracts a street full of zombies to follow her. While she
arrives home, she meets her boyfriend, Mike and his brave but idiotic
best friend, Terry. A zombie tries to break in but Mike kills it. In
the process, Mike becomes bitten. At that time, Michonne bandages
Mike, who is unaware that he is infected. That night, Michonne
searches a neighbor's house for supplies and discovers her trademark
sword. She gets pinned down in the house during the night and returns
home the next day, finding that both Mike and Terry have become
walkers. She manages to survive their attack and lock them away.
noisy encounter causes her street to become overrun. Michonne becomes
trapped and runs out of food. She notices walkers don't attack one
another and hacks off the arms and lower jaws of Mike and Terry. She
chains them up by their necks to use them as escorts for an escape
that was successful. Michonne says her final words to the zombified
Mike and Terry, to help remember who they are, and because she has no
one to talk to.
Michonne on Walking Dead Wiki
Here's an image I found with Michonne fighting off the two of them: