When an activity is published and participants are invited - you can communicate with participants via posts on the activity.
Click into the activity when you are in the "Calendar" tab in the app to get an overview of the activity. If you click into "Posts", you will see the messages that have been added.
Here you can like, comment and share others' posts or create your own. To create a new one, press the red "+" button in the bottom right corner.
Here you write the post and press "Publish" at the bottom to post the message.
When a post has been published, all participants will receive a notification in the notifications tab that something has been posted in the activity.