Hey Gary,
You are not the only commissioner to ask about how to send a reminder email to slow-moving members of their league to get them to rejoin, especially if they have a hidden email.
You can always send a reminder email by using the Invite Friends tool, which is detailed in this Help article:
Inviting friends to a league
You're right, though, that things get a bit dicier when a member chose to hide their email address last season, meaning their email address isn't visible in the tool this season.
Per our Privacy Policy, we can't release these email addresses for any reason.
However, if you look through your league archives, you may find a season in which that owner didn't hide their email address. You can copy and paste the email address from the archived page, then enter it into the Invite Friends tool for this season.
I really hope you find this information helpful, or that it resolves your issue completely. If so, please be sure to mark it as such, so others can benefit from this reply.
Good luck this season!