While there is a history of atheist/agnostic thought within the Hindu philosophical tradition, have there ever been actual communities of atheist/agnostic Hindus?
Yes. There is a system of philosophy known as Lokayata which supports atheism. The followers of this system are known as Charvakas. Bruhaspati founded this branch of philosophy. Charvakas do not believe in Heaven, Hell, Soul and Karma
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.