did you find that option so far ? I'm struggling with same damn issue!?
Thank you for the message. Please go to menu Home - Family members - then please click on an option More by the member you want to promote to webmaster.
There are settings to allow guests to update your site too.
Don't ask me what they are, as I've been trying to turn them off!!
I've had people updating thier families details and can't tell what they've done.