Phonosemantic compound. 糹 represents the meaning and 勺 represents the sound. Based on the original meaning "bind". The meaning later shifted to "agreement", "restrain", "keep under control", "appointment", and "approximately".
約 and 勺 don't sound similar in modern Mandarin due to historical phonetic changes. They were more similar in older Chinese.約 /*[q]ewk-s/, /*[q](r)ewk/ 勺 /*m-tewk/, /*tewk/
treaty, agreement, covenant; to make an appointment; to invite; approximately; pact; treaty; to economize; to restrict; to reduce (a fraction); concise
