You can add a user to a particular interest group in a couple of ways:
  1. Create a custom constituent import and include "Interest_ID" as one of the columns.  Details on Custom Constituent Imports.
  2. Add the constituent to the interest group through their Constituent360 profile
    1. Search for the user in Constituent360 and View the record
    2. Select the Email Preferences tab
    3. Click "Edit Site-Wide Preferences"
    4. Add the user to the interest by changing their status to subscribed.