In this chapter, we extend several classical social choice functions, including nonranked social choice methods such as plurality and majority and ranked social choice methods such as Borda count and Condorcet method, to signed and weighted social influence functions (rules) in the context of group decision-making with mutual influence, respectively as: plurality influence rule, majority influence rule, Borda influence rule, and Condorcet influence rule. Social choice functions take individual choices/preferences as input and give a collective choice/preference as output; likewise, social influence functions take influencing agents’ choices/preferences as input and give the influenced agent’s choice/preference as output. We provide an example of group decision-making with multiple sources of influence to apply and illustrate social influence functions we design. Social influence functions can be understood as the mathematical expressions of social influence rules.