Atheism has absolutely no connection whatsoever to satanism, and only a passing relation to agnosticism.

Atheists tend to define atheism as the lack of belief in any god or gods. Since a self-definition is the most likely to be accurate, one need not bother to seek defintions of atheism by non-atheists, as such definitions are likely to be the result of lack of sufficient knowledge, or worse, a bias against atheism. There is also a stronger sort of atheism in which one actively denies the existence of G-d, or god, or gods, whether a specific one, or in general.

- quote (http://atheism.about.com/od/definitionofatheism/a/definition.htm ) -
The broader, and more common, understanding of atheism among atheists is quite simply "not believing in any gods." No claims or denials are made — an atheist is just a person who does not happen to be a theist. Sometimes this broader understanding is called "weak" or "implicit" atheism. Most good, complete dictionaries readily support this.

There also exists a narrower sort of atheism, sometimes called "strong" or "explicit" atheism. With this type, the atheist explicitly denies the existence of any gods — making a strong claim which will deserve support at some point. Some atheists do this and others may do this with regards to certain specific gods but not with others. Thus, a person may lack belief in one god, but deny the existence of another god.
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Agnosticism is defined as lacking knowledge about the existence or lack thereof of G-d, or god, or gods, and the assertion that such is intrinsically unknowable. An excellent article is available at http://atheism.about.com/od/aboutagnosticism/a/what.htm which quotes from Gordon Stein's essay “The Meaning of Atheism and Agnosticism”:

- quote (Gordon Stein) -
Obviously, if theism is a belief in a God and atheism is a lack of a belief in a God, no third position or middle ground is possible. A person can either believe or not believe in a God. Therefore, our previous definition of atheism has made an impossibility out of the common usage of agnosticism to mean “neither affirming nor denying a belief in God.” Actually, this is no great loss, because the dictionary definition of agnostic is still again different from Huxley’s definition. The literal meaning of agnostic is one who holds that some aspect of reality is unknowable. Therefore, an agnostic is not simply someone who suspends judgment on an issue, but rather one who suspends judgment because he feels that the subject is unknowable and therefore no judgment can be made. It is possible, therefore, for someone not to believe in a God (as Huxley did not) and yet still suspend judgment (i.e., be an agnostic) about whether it is possible to obtain knowledge of a God. Such a person would be an atheistic agnostic. It is also possible to believe in the existence of a force behind the universe, but to hold (as did Herbert Spencer) that any knowledge of that force was unobtainable. Such a person would be a theistic agnostic.
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Satanism is usually defined as worship of Satan, however, some Satanists disagree with that definition, and instead claim it is merely having a favorable view of Satan. According to a section quoted by http://theisticsatanism.com/scholar/definition.html from http://theisticsatanism.com/bgoat/who.html :

- quote -
a Satanist is anyone with a favorable view of Satan. We define "Satanism" not as any one particular belief system, but as a general cetegory (sic) of belief systems all involving a favorable view of Satan. There are many different kinds of Satanism, or, one might say, many Satanisms. A theistic Satanist is one who reveres Satan as a deity. Other Satanists we refer to as symbolic Satanists. A symbolic Satanist does not believe in Satan as an actual entity, but, instead, regards Satan as a symbol of values which a given particular Satanist endorses, usually including individuality and independent thought.
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As can be seen, atheism has nothing to do with Satanism, though it is possible for someone to be an atheist and a symbolic Satanist. However, it is as possible for a theist to be a symbolic Satanist. Being an atheist means not believing in any deities, so that would automatically make it impossible for an atheist to be a theistic Satanist. If anything, a theist is more likely to be a Satanist than is an atheist. Finally, atheism did not evolve from Satanism, nor is the reverse true. The two are unrelated.

I really hope that this question was a joke, but sadly I doubt that it is. For starters, there are two branches of Satanism (LaVey and Crowley). LaVey Satanists can be atheists because those Satanists don't actually believe in a literal Satan. They just use that aspect of Christian mythology as a metaphor. Originally, the character of Satan in the Bible wasn't an evil being, he was just "The Adversary" to the character of God. This was similar to a court of law in which there is a prosecutor and a defense attorney. This is most clearly depicted in the Book of Job. So in LaVey Satanism, Satanists are called to play "Devil's Advocate" in the sense that they challenge the status quo.

Crowley Satanism is a bit different. These Satanists do believe in some sort of supernatural being but it isn't necessarily the Christian Satan. So these Satanists would probably not be atheists.

Atheism is a lack of belief in a deity. So if an atheist doesn't believe a god exists, he or she almost certainly doesn't believe that The Devil exists either. In fact, most atheists reject all these supernatural beings.

As it turns out the only people who even believe that Satan is real are Christians. So when you meet someone claiming to be a Satanist who actually worships Satan, they are probably just a rebellious Christian.

Yeah, there can't be "Satanism" unless there is religion. Typically, within a religion, an ANTI-religion figure of some sort is used as a definition of failing to be religious.
Atheists do not have a deity to "worship", so by reason, they have no counter to a deity.
I'm not knowledgable about religion so I'm not knowledgable about the Christian idea of "Satan" either, except what I see in other fiction such as horror movies. I've lived all my life among non-religious people and have never heard of anyone even mentioning an opposite to a deity. Even little kids dress up as devils on Halloween, so how serious can the concept be?