The phrase "joined in" can be replaced with a number of synonyms depending on the context of its use. For instance, it could be used in the context of a group activity, in which case "participated in" or "took part in" could be used instead. In a more formal setting, "contributed to" or "collaborated on" would work well. Other potential synonyms for "joined in" include "joined forces with," "united with," and "teamed up with." It's important to consider the tone and intent of the language being used when choosing a synonym, as some options may have different connotations or levels of formality.