join of users and users_groups as 1 row

You will need to do a GROUP_CONCAT but Codeigniter active record has no GROUP_CONCAT method.

The data returned should be a string. You will need to do explode() so that it becomes an array.

GROUP CONCAT http://stackoverflow.com/questions/27692...-one-field
Manually write SQL query in active recordhttp://stackoverflow.com/questions/17167573/concat-in-codeigniter-active-record