Most of my friends are extroverts.
I am  definitely an introvert, not outgoing at all. 
I think I am probably in between.
So you don't like being alone but are good in crowds? Or how do you characterize your introversion/extroversion?
It depends on the situation. I am more comfortable in smaller groups of close friends.
I guess a lot of people feel they have a mix of both personality types.
That makes sense.
Also most people think that there has to be one side that is more dominant than the other.
I could see that being the case. It does seem like a person would lean more one way than another.
defined introversion as an "attitude-type characterised by orientation in life through subjective psychic contents" (focus on one's inner psychic activity) and extraversion as "an attitude type characterised by concentration of interest on the external object" (focus on the outside world).
Varieties.
Varieties Extraversion.
Extraversion (also spelled as "extroversion") is the state of primarily obtaining gratification from outside oneself. Extraverts tend to enjoy human interactions and to be enthusiastic, talkative, assertive, and gregarious. Extraverts are energized
are growing uneasy. What lands you have, you may retain forever; but we shall sell no more. You think, perhaps, that I speak in passion; but my heart is good towards you. I speak like one of your own children. I am an Indian, a red-skin, and live by hunting and fishing, but my country is already too small; and I do not know how to bring up my children, if I give it all away. We sold you a fine tract of