At a more Informal party.

When you introduce two of your friends to each other,you can simplify say,
"John, this is Sarah".
Cultural considerations:
In a work situation, there's often one person who has lighter status -your boss,or a client,for example,In these contexts,it's polite to address them as "Mr..."until the situation becomes more informal.
                                   However,some people will say to you,"Please call me(Henry)",and you know that you can use first names.If someones call you by your first name as soon as they meet  you,then you can use their first name too..
                                                What do you call women?Someone prefer not to call women by their names at all,until the situation is more informal.Of course friends,colleagues and family will always use first names....
                                          Men and women in European and English-speaking cultures often shake hands,when they meet someone for the first time.........