Complex sentence: Social identity theory suggests that people identify with groups in such a way as to "maximize positive distinctiveness"; groups offer both identity (they tell us who we are) and self-esteem (they make us feel good about ourselves).
Split into simpler sentences: Social identity theory suggests that people identify with groups in such a way as to "maximize positive distinctiveness". Groups offer both identity (they tell us who we are) and self-esteem (they make us feel good about ourselves).