I have already put the game in the Adult category but on the suggestion of my lone player would like to make it private as well. Thank you in advance.

You can change the thread group! Groups 0 and public are visible by people not in your game, while others are visible only to players assigned to the same group.

Go to your GM menu in the top right, select "edit groups," and assign your player to whatever groups you want (I usually start with 1 or A). Then click into the thread, choose the matching group from the dropdown menu, and update!

I use specific group letters for certain purposes. For instance, I tend to use Group R for any kind of Reference threads, Group S for anything about the game's Setting, and Group T for any Tables I need. If the players have any personal threads for their own reference (like, say, giving one player a thread for them to copy the info for their spells), I usually put that in Group U as it's for Utility threads.

It's not strictly necessary to go to this much trouble, but I find it helps me stay organized.

Keep in mind for adult games even Group 0 is blocked by the age filter. The old rules where you could get slapped by am admin for making adult stuff public doesnt matter anymore but Ive found a lot of users don’t know that and still panic if they see a group 0 thread in an adult game. Also quite a few GMs still asking for age verification PMs which are obsolete as well.

In reply to praguepride (msg # 4):

But also keep in mind that the rule about "no Adult content" still applies to character description pages, and the game intro.  Those areas are still public, and must be suitable for all.