In mason we used to display "charter member" if we didn't know the actual date the user joined. e.g. http://classic.musicbrainz.org/show/user/?username=ModBot and http://classic.musicbrainz.org/show/user/?username=Hawke

Now these accounts show up as being registered at 2000-09-07 00:00:00 UTC e.g. http://musicbrainz.org/user/ModBot and http://musicbrainz.org/user/Hawke

I seem to recall that we invented dates for people who didn't have them. I think we should still display something different for the accounts that applies to. I'd suggest "since the early days of the project" for the wording since it's clearer than "charter member" and is similar to what's displayed for rob - http://musicbrainz.org/user/rob