Tick box sounds good.

Should it be opt-out of anonymity or opt-in?

I think having the possibility to not be anonymous would ruin it for the team. Some people might want to show how much they love the company (honest or not) by switching of anonimity. The anonymous group would thus become suspicious.

opt-out of anonymity in my opinion, always default to the safe choice. As your application grows you can make this a setting the manager decides.