The participant lists currently sort by join order, which seems less useful than sorting alphabetically. For example, it makes it more difficult to quickly assess if a specific user is a room member.
Revisions and Commits
A better description of this task is probably "Participant lists should have a meaningful, obvious ordering".
If they already have a meaningful (but not obvious) ordering, we should make it obvious (e.g., if it's by-last-message, put "last message X minutes ago" next to each entry or whatever).
If they don't have a meaningful ordering, using alphabetical seems reasonable, possibly with the "you on top" and/or "admins on top" rules.
The existing order is based on when participants are added. (I think anyway; the query is on conpherence phid and the index used is (conpherence phid, participant phid) so I think it ends up ordered by id? Seems like that on my test data.)
Ergo, go with the "I am at the top, rest of the list is alphabetical" option. Diff in a moment.