XkbSAGroup returns the group_XXX field of act converted to a signed int.
Actions associated with the XkbGroupAction structure change the current group state when keys are pressed and released:
    typedef struct _XkbGroupAction {
        unsigned char    type;       /* XkbSA_{Set|Latch|Lock}Group */
        unsigned char    flags;      /* with type, controls the effect on groups 
*/
        char             group_XXX;  /* represents a group index or delta */
    } XkbGroupAction;
   
 
This manpage seems to be incomplete