السلام عليكم ورحمة الله وبركاته If I miss ‘Isha in congregation at the masjid, is it better to pray later in the night, or to make a new congregation with my friends?

If you're going to pray later and you won't forget in congregation, then yes. However, you do know that praying it in the masjid with the congregation is better than delaying it and not praying it in the masjid.