If you want to automatically notify another Tracker user during the note taking in either an Activity or a Note, you can simply @mention them as you type.
To @mention another user, type "@" and the start of the name of the user. This will present you with a list of users who's name matches those letters:
Once selected, the user will appear as a bubble in the text to make it clear that they have been mentioned:
On saving the Activity or Note, the person (or people) you have @mentioned in the text will received an on-screen alert informing them that they have been alerted and the message that they were alerted in, so they can action accordingly.
Note: The text for the @mention will be replaced with the plain text version of the name when saved to Tracker.