1. No matter
This is another way to say, "It doesn't matter" or "Forget about it." It is short and quick and preferred during casual conversation. This phrase is almost always followed by an alternative.
2. Count me in
You can include me in your plans because I will definitely come. It is like counting the number of voters in a room. The ones with the hands up high are included in the count.
3. What was her name again?
You could have said, "I have forgotten her name, what was it?" The use of "again" assumes that you have forgotten. This is a common way to ask about things that you should know but has skipped your memory.
4. You would look at her
This phrase insinuates that a person who was expected to do something goes ahead and does it anyway. In the dialogue, for example, Sergio is focusing on the bridesmaid because she is the most interesting part of the wedding to him.