The only religious organization in the USA that have "authority" over its congregants is islam (and this is theoretically voluntary as well).
The rest are groups of people who CHOOSE to worship together and CHOOSE to behave (or not) in ways established within the theological framework of the religion, church, synod, etc.
For example, so few Catholics adhere to the strict guidelines of the Church that they have become mere suggestions (God may weight in later but that is neither here nor there).
Atheist organizations are numerous, some have HUGE participation. Atheist organizations have congregants just as estabish theistic religions do.
Besides, most atheists do NOT not believe in God. They just hate Him.