A number of canons speak about not praying with heretics, Jews, etc., going to their Churches, and so forth (Apostolic Canons 45 and 65 for example). However, how and when and why canons are applied are the prerogative of bishops. There is also a canon against going to Jewish doctors, for example (Quinisext, Canon 11), but few, if anyone, follows that canon today. That doesn't mean that the canon is obsolete, it simply isn't applied or used in the same way it was 1,300 years ago: it's purpose that it served then is different than the purpose it serves now. So to with the 15th Canon of the First Ecumenical Council, among others. These canons generally aren't in active use, though that doesn't make them useless. The underlying spiritual reasoning is still in place, it's just that the application has changed. With that in mind, if your bishop says it's ok to go to a Lutheran wedding, or to pray with other Christians at a sporting event, or to attend a Catholic Bible study, or whatever, then that's the authoritative/definitive answer.