The phrase "in league" means to be in partnership or collaboration with another person or group. There are various synonyms that can be used in place of "in league" depending on the context. Some of these include allied, cooperating, collaborating, working together, in cahoots, and in collaboration. "Allied" implies a mutual agreement or partnership while "cooperating" suggests a willingness to work together. "Collaborating" suggests joint efforts to achieve a common goal. "Working together" emphasizes teamwork and joint efforts while "in cahoots" implies a secret or dishonest partnership. "In collaboration" suggests a positive and deliberate partnership for a specific purpose.