Are their any branches of atheism? Like how the orthodox church branched of from the catholic church?
No.
Though from my understanding most atheist do not belong to formal groups, much to our disadvantage.
I don't see why, atheists shouldn't really feel the need to spread atheism or promote it. Neither should religions. As long as atheists aren't having their freedom infringed upon, I don't see any reason why they should need to tell me what they think of religion and seeing as that's pretty much the only thing they would do as part of an organisation, they don't really need an organisation.
Emphasis is mine. This is why it is to our disadvantage about being a group of individuals instead of a group with a purpose. There are more atheist in the US then several major minority groups. Our voice (as scatter as it may be), is underrepresented on all civil levels.
This means that we get government supported religious actions. Like funding religions for humanitarian works with no oversight, special treatment for private religious clubs like the Boy Scouts of America and again special funding. State level governments using public funds to refurbish private own religious centers. City councils have sectarian pray sessions, open atheist city council member being wrangled out of their appertained seats.
Or having the blasted ten commandants at the front of court houses, when a majority of those laws (with there many flavors), are illegal.
Our (US) national motto changed to 'In God We Trust', and having it appear on the money in civil buildings.
Let alone the countless laws stating that atheist can't vote, can't hold private office, can't own land, can't serve as a police offer, fire fighters so on. Thankfully these laws are powerless, though it is a standing reminder of atheist mistrust.
I don't think spreading atheistism, is a terribly good thing as stated before. Unfortunately, atheist need to be popularized simply due to the, for the lack of a better word, because religious recruitment for members.