If what you're looking for is true closure to a time in your life that you no longer have the need to revisit - then I say let the other active members know the site will be closing on a set date - and let them make arrangements to stay in contact however they see fit. You're not bound to this group for the rest of your life, everything has a purpose and a season, and maybe both are over this group. If keeping the site open and relinquishing control will not allow you the full closure that you need - then that is not the best option for you. Anyone of them if fully able to set up a new group or start a new blog or website.
I say - let it go and move forward with your life. And fwiw............ I'm very proud of you for knowing its time to stop looking back, and face forward! Good job!