one of the documented procedures in this installation of the ACS
events_group_add_user   db   event_id   user_id
What it does:
Add's user_id to the user group of event_id's event. Returns 1 if successful, else returns 0.
Defined in: /web/philip/tcl/events-defs.tcl

Source code:

    set group_id [database_to_tcl_string_or_null $db "select group_id
    from events_events
    where event_id = $event_id" 0]
    if {$group_id == 0} {
	return 0

    set return_id 1

    if [catch {set return_id [ad_user_group_user_add $db $user_id "member" $group_id]} errmsg] {
	set return_id 0

    return return_id