As you can see Demon Hunter and Crusader are the only two classes that had 0% or close to 0% participation in group play for 4 ERAs in total, last 4 ERAs in row. This are also the ERAs with highest group clears. Demon Hunter was dominant in first two ERAs and participated in 3rd Era fairly but Max group Clears in first two eras where 64 and 67 respectively which is nowhere close to what groups where able to do later (130 GR clears in ERA 6 for example). Season 3 ended on August 23 2015 and if I remember correctly ERA 3 ended on same date. So when ERA 7 ends it will be way more than a year since DH and Crusader had their spot in 4 man groups.
Witch Doctor is the only class that had its spot in 4 man groups each ERA. Just like Demon Hunter, Witch Doctor was never the best solo clear class.
Monks and Barb had 0% or close to 0% participation in group play in first 3 ERAs but later they had constant spot in groups that gained most Paragons by clearing 100+ GRs. Barbs where dominant in solo play first two ERAs, specially first ERA when they where able to clear GR 60 which is only 4 levels below the best 4 man group with two Demon Hunters.
Wizard had insane 95% participation in solo rankings for ERA 6. They where also the main DPS in groups in ERA 5 and ERA 6 when we saw higest GRs cleared in history of Diablo 3. Three ERAs Wizard was the best solo class. They still see some above avarage use in groups for ERA 7.