As for занимались/занимался, let me quote Wade. The section is quite long I'll give you only the relevant part.
===[Start Cut]===
When, however, there is a plural antecedent (все or те), кто may take either a singular or plural verb: Все, кто пришёл/пришли на собрание, голосовали за меня. 'Everyone who came to the meeting voted for me.', cf:
Среди тех, кто остался, был Иван Карлович, наш сосед (Rybakov) Among those who remained was Ivan Karlovich, our neighbour
and
Те из нас, кто читали стихотворение, были в восторге Those of us who read the poem were delighted ===[Endt Cut]===
I also looked up Cambridge "Using Russian, a guide to contemporary usage" by Derek Offord, and a few other books, but was unable to locate a related discussion.
no subject
As for занимались/занимался, let me quote Wade. The section is quite long I'll give you only the relevant part.
===[Start Cut]===
When, however, there is a plural antecedent (все or те), кто may take either a singular or plural verb: Все, кто пришёл/пришли на собрание, голосовали за меня. 'Everyone who came to the meeting voted for me.', cf:
Среди тех, кто остался, был Иван Карлович, наш сосед
(Rybakov)
Among those who remained was Ivan Karlovich, our neighbour
and
Те из нас, кто читали стихотворение, были в восторге
Those of us who read the poem were delighted
===[Endt Cut]===
I also looked up Cambridge "Using Russian, a guide to contemporary usage" by Derek Offord, and a few other books, but was unable to locate a related discussion.