The antonyms for the phrase "joining up" are dissociating, disconnecting, separating, and disuniting. Dissociating refers to the act of separating or breaking away from a group or an alliance. Disconnecting implies the cutting of connections between two or more entities, and separating means the physical removal of one entity from another. Disuniting is the act of breaking apart or dissolving a union or a relationship. In contrast, the act of joining up involves bringing together, forming a connection, or joining forces with something or someone. The antonyms provide an opposite meaning to joining up, suggesting detachment or breaking apart.