The badminton club had so few serious members that it was basically either disbanded or on hiatus.
Literal
Badminton-club [topic-は] proper [copula-な] member [subj-が] extremely few [because-ため], almost dissolution-or-suspension state [copula-だった].
まとも 'proper / serious / decent' (as a na-adjective) qualifies 部員 'club member' — 'members who actually show up and do the thing.' ため 'because / for the reason that' is a formal causal. ないし (literally 'if not') is a formal conjunction meaning 'or / either X or Y,' slightly stiffer than または. 廃部 'club dissolution' and 休部 'club hiatus' are compact sinitic compounds for school-club organizational states.