While there is a history of atheist/agnostic thought within the Hindu philosophical tradition, have there ever been actual communities of atheist/agnostic Hindus?
There have been 4 main non-theistic(nirīśvara-vāda) schools in Hinduism:
- Purva Mīmāṃsā
Cārvāka was a materialistic non-theistic school. I.e they believed in only pratyakṣa(perception) pramāna(valid means to knowledge). This puts them in the material realm. A simplified view of their philosophy would be : "If you can see something which exists, it must be true or else it is false. I cannot see Brahman, so Brahman does not exist. I can see this tree, so this tree exists."
The first 3 are spiritual non-theistic schools. I.e they believe in more than just perception and extend valid means of knowing to meta-physical realms. These are similar to Buddhism & Jainism in rejecting a creator God but not rejecting the meta-physical realms which are beyond human perception(similar only in this aspect).
Cārvāka is not usually grouped along with Sāṃkhya, Purva Mīmāṃsā & Yoga despite all the 4 being non-theistic is because Cārvāka doesn't consider śabda pramāna(Śruti or Vedas) to be valid means of knowledge, while the others do. They(the former 3) interpret Vedas non-theistically, mainly as a source of ethics(Karma, Dharma) only and don't believe in creator & sustainer God.