checks if user group record already exists & prevents duplicate
added add_to_group() documentation to user_guide
add a function to get user id from session - this is always the first thing I add to ion_auth when I start a new project
isolates the session set as a function, so you can use the trigger_event() to add additional session vars (or anything else)
corrected names of hooks for set_session function
get_user_id() returns null, not 0, when no session
Merge pull request #388 from jrmadsen67/2

checks if user group record already exists & prevents duplicate