It's very important to companies whether the students who they gave offers to will actually join or not
It's very important to companies
会社にとって重大だ
the students who they gave offers to
内定を出した学生
theyはthe companiesという意味で理解されます。
内定、は英語でjob offerで表現できます。
whether (the students) will actually join or not
（学生が）本当に入るかは
It's very important to companies whether the students who they gave offers to will actually join or not
内定を出した学生が本当に入るかは会社にとって重大だ
ご参考になれば幸いです！
アメリカ合衆国 
