The user id that you would have in any table other than the user's table would technicallly be a foreign key, but it is not necessary to define this in the table, you can just have it be another column in the table.
How you go about associating a record with a user is to ensure that your login server behavior is storing the userID in a session variable. When you insert a record into a table where you want also record the user you will use the DataAssist insert server behavior, in the list of column select the userID column in this table and use the lightning bolt icon to associate this field with your userID session variable. This goes for any table that you have the userID column added to associate those records with a particular user.
When it comes to updating and deleting after these pages are created by DataAssist you will want to update the recordsets on these pages. You will need to add in a where clause and say to make it select the records where the userID is equal to the session variable for the logged in user.
If you have any trouble with any part of this let us know and we can provide more info or details depending on what you need.