I would like to contact the participants of a project directly?

As an admin, you will see the “Manage” item in the left sidebar. Here is an overview of all active and previous projects. Go to the project for which you want to write to all participants. Under the project image, you can see the points: “Notification,” “Reminder,” and “Urgent Call.” The text is pre-filled here by default. However, you can rewrite it for your own purposes. Attendees will receive your message as an email from the Lets Team (or via Slack or Teams if you've set up an integration).