When you have a valid booking for an activity, you will be given access to the Activity Message Board. This allows you to communicate with other Participants on an Activity!
A User can be tagged in a note on the Message Board by simply adding an "@" along with their username.
For example, if you wanted to tag a User with the username “BilbyUser99” you could simply add a message with:
Hello @BilbyUser99, how are you? :)
This would then reference that user in the note. They will also receive a notification if they are subscribed to that notification.