There are plenty of synonyms for the phrase "signed on with" that can be used to indicate joining a group or organization. Some examples include "enlisted with," "aligned with," "committed to," "associated with," "joined forces with," "partnered with," or "became a member of." These phrases all express the idea of making a commitment to a certain group or cause. Whether it's joining a sports team, becoming a member of a professional organization, or aligning oneself with a certain political party, there are many ways to express the act of "signing on" with a group or organization.