An E2EE adversary is able to intercept, read and modify any messages sent over the network, and has full access to the messaging server (bypasses the client-to-server encryption). An E2EE adversary like that is assumed to have a very strong computational power, such as NSA and GCHQ.
A malicious user and a malicious group member can collude with a E2EE adversary, or an E2EE adversary him/herself can be a malicious user and/or a malicious group member.
I wish I could write only dictatorial governments are adversaries, but ghost stories tell tales of state run Man-in-the-Middle attacks in which secure messaging platforms such as WhatsApp and iMessage can either be forced to create backdoors or to operate a ghost protocol that “CC’s” an encrypted message to a third party when it is sent. The legislation for it seems to be well on its way in the Common Wealth (Five Eye) countries, and is likely to be adopted/copied by other countries (if not already in force in dictatorial run countries):
In December 2018, Australia’s parliament passed legislation that allows intelligence and law enforcement agencies to demand access to encrypted communications data.
The GCHQ disclosed late 2018 an intention to “silently add a law enforcement participant to a group chat or call”, according to the “Principles for a More Informed Exceptional Access Debate” article written by Ian Levy and Crispin Robinson, enabled by the UK Investigatory Powers Act (IPA).