Add a user using the '+' button in the action bar.
Enter the users bitcointalk UserId
To find the UserID browse to the bitcointalk profile page of the user you want.
Look at the URL in the address bar for 'u=XXXX'. The number in place of XXXX is the userID.
For example Satoshi Nakomotos profile as at this link:
At the end of the link you can see "u=3" so his UserID is "3"
Once you have selected the users you want. Close the app and you will start receiving notifications for those users.