Not sure whether to report this as a bug. There used to be a search icon on the Participants screen where you could filter which Participants to add to a survey. I synced the latest Yii branch from Github and noticed it's not there anymore.
Well, in case anybody's wondering, I did find the search function. The icon changed from a magnifying glass to a caret ^. Maybe that was a design error? The magnifying glass is still used on the Attribute management screen.