Usage patterns
〈事〉の党員になるto become a member of a (political) party
He became a member of that party last month.
党員として〈動詞〉to act/speak as a party member
She took part in the election campaign as a party member.
党員数が増える/減るthe number of party members increases/decreases
In recent years, the number of party members has been decreasing.