what's the meaning of "a roaster of" in this sentence?

It's not a roaster of, KK, it's a roster. Note that you have added an 'a'.

A roster is a list of people that work for a company/government agency, play for say, a baseball team; a roster [no 'a'] would include all the players who play for that team.

Thank you very much. I should have been more careful

No problem, KK. It could happen to anyone.