I am unable to get this to work -- it actually works fine in PREVIEW, but not at all once published. I have a lot of other Contact Selectors in this form, as well as other Contact Selectors within repeating tables in this form.
For a non-optional contact selector, my setup is: Repeating Group (Repeating Table ( fields ... (ContactGroup (Person ( person fields ) ) ) ) )
The optional section version will just have an extra Group corresponding to the Optional Section containing the Contact Group stuff.
Once published, clicking "To..", typing in names, hitting the check button.. none of that works for the optional section selector.
I have considered moving the Opt sect group out of the repeating table group.. I think that would make the selector work, but I don't think it would be 'repeating'... not sure.