联盟(lián méng) - alliance, union, league in Mandarin

联盟 (lián méng) is a formal alliance between parties that stay independent: states, political groups, companies, sports clubs. The 盟 half carries an old sense of a sworn oath, which is why the word feels weightier than a simple partnership.

Typical verbs around it are 组成 or 成立 to form one, 加入 to join one, and 退出 to leave one. The measure word is 个.

The trap is stretching it to cover any group. A union in the labour sense is 工会, not 联盟, and a club is 俱乐部. It also sits close to 同盟, which is largely interchangeable but sounds more historical and military. And note the near-twin 联系, to contact or connect: the shared first character misleads plenty of learners, but 联盟 is a body you belong to and 联系 is something you do.