how can i turn sharing on for a reminder after it is created

Hi phuff530

You can change the list that a reminder belongs to. If you created the reminder on a non-shared list, you can move it to a shard list by:

On OS X: Just drag the reminder to the shared list you want in the column on the left

On iOS Device: Tap the "i" to the right of the reminder you want to move then tap List, select the shared list you would like to add it to then tap Done on the top right of the screen