To answer your question, 'members' are users that have a membership in the room (used exclusively for private rooms) and 'participants' are users that are currently in a room.
I think 'participants' is a handover from the HC Cloud architecture, which is exclusively used for caching.
Hope this helps!
HipChat Server Support Engineer | Atlassian
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...able to use the clients After a bunch of testing (rooms, memberships, check for private message history, and most importantly - the custom emoticons!) nearly everything was looking as we needed, however...
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