Husband joined a singles group on FB

I got on my husbands Facebook yesterday evening as I have done occasionally when he leaves his phone. Well, one of his friends sent him a suggestion to join a singles group so I clicked on groups and sure enough he is now in that group for single women and men. I’m not sure what to do about this. Should I wait and see if he actually talks to anyone on the group? Should I mention something and admit I was on his phone? What should I do?