"Belonged to" is a popular synonym for the phrase "were a member of". Other options include "part of", "enlisted in", "joined", "affiliated with", "associated with", "connected to", and "included in". These synonyms can be used interchangeably, depending on the context and the tone of the sentence. Using synonyms can help vary your language and avoid repetition, creating a more engaging and interesting piece of writing. Whether you're writing an essay or a professional email, using synonyms can add depth to your language and make you sound more skilled and articulate.