Don't know what Garmin did to the its software, but Group Track was finally working last night! Before that I was trying for six months without full success. In the past, only certain combo works, like Edge 820, 520 plus Fenix 5 works together, but not with 830. 830 works when paired to Samsung phones but not Huawei, etc. It was really frustrating.
Then last night I decided to give it another go, I asked my cycling buddies, with Fenix 5, 830 and 520plus, to switch on Group Track and behold! - one by one each appeared in the group track connection data page.