‘Is Lisa here?’ ‘Yes, I’ll get her for you.’
The verb to get, as you may already know, can have tons of different meanings depending on the context. In this particular situation, it means to manage to "obtain" something, or more precisely someone. See definition #2 in the English Oxford Living Dictionaries: succeed in attaining, achieving, or experiencing; obtain.
So, in your example it's saying that you're going to go and find Lisa to let her know that somebody is asking for her. The most likely thing that will happen after you finally get ahold of her is that you're probably going to bring her over to the person who wants to see her.