It depends on whether the market is a qualifying SWFA market, yes. I believe you can qualify for an associate membership with one short story sale in a qualifying market and a full membership with three short story sales, but I'm not certain where novellas fall on that scale. I'd guess they'd be treated as short fiction for the purposes of SFWA membership, but that's, well, a guess.
ETA: SFWA's membership eligibility guidelines and list of qualifying short story markets.