may kabuto,hinata,sai,karin,kankuro,kakuzu and the other...if you follow that order. kabuto is an essential,and you may upgrade you hinata fast to 4*, Sai is good in my opinion, kakuro too, kakuzu is good for combos.

Kabuto, hinata make them 3* asap, then get sai. Kankuro is good but you can sweep elite instances for him, tobi is only good if you want to use him, you will get karin from free draw, Kakuzu - very less people uses him, too easy to counter. Focus on getting sasuke 3* asap.

Buy medium refines from group shop and low level refines from all the events, try collecting common thread from events when they are cheap points for free.

Easiest way to farm group coins: attend all great ninja war round and daily mission. Stack as much refinement you can.

Thanks for the tip.
I will work on kabuto, honata and sasuke.
Also, where do i use the refinements?

strengthen menu - equipment refining.