There are 2 logical ways to approach this:
1. Lie. Tell them that their account has a virus, and you need the password of their account to get to the root of the problem (as if the virus was on their account).
2. Tell the truth. This seems like the most logical approach. Tell them that you are auditing the network, and need their password for some urgent reason. Give them your credentials (but probably should give a fake name), what does it matter? If someone else was breaking into the network, they would give fake credentials, so why not supply the most authentic ones you can get your hand on (your own). If you convince them you were hired by the company, then you succesfully social engineered the password.
Just some ideas...