You sure can! We call this '@ mentioning', because you place the @ symbol directly in front of the username of the member to whom you wish to address your message. To do this, you'd format the message like this:
Hello @teammate1, @teammate2, @teammate3, how are you?
After you click or tap the 'post' button, it will look like this:
You'll notice that a some of the usernames aren't visible but they will receive a notification, and you can select those usernames to make sure you've mentioned everyone you want to mention. When you select those usernames, it looks like this: