When you're describing someone's actions or activities, there are plenty of synonyms you can use instead of "are engaged in." For example, you could say they're "taking part in," "involved in," or "embroiled in" something. Likewise, you might explain that they're "occupied with," "devoted to," or "committed to" a particular pursuit. Other options might include "focusing on," "working on," or "immersing themselves in" a project or task. Ultimately, the key is to find a word or phrase that accurately reflects the subject's level of involvement without becoming repetitive or cliched.