A film isn’t a film until it’s shared with an audience. Your presence is required. Touch base with your prof who will register your attendance.
Yes, you may (and should) invite friends and family.
Take note of AUDIENCE RESPONSE to your film. Did people react when and how you thought they might?
Yes, there will be a reception after the screening. Think of it as an opportunity to rehearse professional comportment for networking social occasions. Make eye contact. Start conversations. Demonstrate your interest in the films of others.
Wear what you might wear to a job interview. Not a prom. Not a masquerade. Here’s some help defining the term BUSINESS CASUAL.