When writing text in Cocoon (either as a manual text update, or in a reply) you can mention another member of the Cocoon by typing their name. After you have written a few letters of their name, you should see their name and profile photo appear above the keyboard. Tap on this to confirm the mention (this will ensure that they get notified).
You can also manually mention someone, by writing "@" following by their first name. Example: @Alex
Mentioning in a Reply:
Mentioning in a Text Update