Broadcast encryption protocol for file sharing service (a-la Dropbox)

I'm building a prototype of a (rather simple) Dropbox-type file sharing service, where users can upload & share file(s) with other user(s).

But here's a feature request:

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  I want file(s) to be stored on the server in encrypted form, and

  
  


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  I do not want to store any keys on the server. (i.e. I don't want to give the service's maintainer the ability to access user's data.)

  
  


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  I don't want to force users to use 2nd (decryption?) password(s) for shared files.

  
  

Is there any cryptographic protocol that fits this task?

My previous read is:

Pattern to allow multiple persons to decrypt a document, without sharing the encryption key?

but access right revocation seems pretty complex in this case.

Broadcast/multicast encryption, maybe? I just don't know where to dig.