If you take part in an activity or organization, you participate in it. For example, you participate in a conversation by listening to and talking with others, or you participate in a sport by joining a team and taking part in practices and games.
You can use the verb participate to mean "be involved in" or "share in." Sometimes it is easy to figure out who participates in something — the runners in a race, the people waiting their turn to audition for a part in a play. Sometimes it isn't so obvious. If two students cause trouble but the teacher punishes the whole class, it might be that in not stopping the troublemakers, she views everyone as having participated — just being there means they were involved.