Ethersphere was created in March 2014 by Daniel Nagy and Viktor Trón (maybe?), primarily as an umbrella for their Swarm development activities. As far as I can determine, it was never a legal entity.
Daniel was a contractor for ETHDEV from October, and Viktor was the first employee of the Ethereum Foundation, being hired by Jeff Wilcke in February 2014.
Here is the Github organization.
Grok says:
The GitHub organization https://github.com/ethersphere was created on March 14, 2014, at 01:37:27 UTC.