Perhaps "oblivious" can sound a little derogatory, even in this very neutral context. Could be - "...in their normal use of NOW and will not be aware of which group they have been enrolled in, unless this has been otherwise communicated."

This information may be useful outside of this specific anon marking context - perhaps we should spin it out to its own page, if there isn't already a NOW Central page we could link to that covers this specifc use?