People work out at a Planet Fitness in Alexandria, Virginia, on Jan. 8, 2024. Leah Millis | Reuters Shares of Planet Fitness plunged more than 30% Thursday after the company reported a drag on sign-ups and trimmed its guidance. The stock was having its worst day ever Thursday, as of midday trading. Even though the fitness company saw 21.9% revenue growth in its first fiscal quarter and same club sales increased by 3.5%, CEO Colleen Keating said the company saw a "slower than expected start from a net member growth perspective." "As a result, we are sharpening our marketing to prioritize capturing demand and driving net member growth," Keating said in a statement. "Additionally, we are pausing the planned national Black Card price increase pending a broader pricing review." The first fiscal quarter is usually the company's peak sign-up period. Keating said "internal and external headwinds" dealt a blow to the company's performance.…