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
...have mention name FirstnameLastname. Fe.: @MaartenCautreels This certainly helps to know who is being mentioned in a conversation. Imagine your colleague sends the following message...
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