Narcissism is a major theme in It's Always Sunny in Philadelphia, as it forms a large component of the main characters' psychologies and a significant factor in their behaviour.
The characteristics of narcissism are grandiose behaviours and fantasies, unreasonable sense of entitlement, exploitation of others, lack of empathy, arrogant and domineering behaviour, vanity, envy, feeling "special" or "unique", requiring excessive admiration and recurring fantasies of unlimited success, power, beauty, brilliance or ideal love.
The Gang is severely narcissistic, with all the characters displaying some or all of the above traits to various degrees. As a result, they are highly noxious, dysfunctional people whose inner dynamics are devoid of any actual friendship or loyalty. Any member of The Gang would happily betray, exploit or manipulate the others or anyone outside of The Gang for their own benefit.