To join in a conversation we try to listen to what the other person or people are talking about. When we talk we try to ask or tell about the same subject. Try to ask questions that are on topic (who, what, when, where, why, and how questions work well). If someone asks you a question it's a good idea to ask them the same question after you give a short on topic answer.
When starting a conversaton it's important to think of subjects that you're comfortable talking about and that they might be interested in. For example talking about grades and clothes is ok if you're comfortable talking about them but if you don't want to be asked about your grades or clothes try to avoid asking questions about these subjects.
To join in a conversation, first listen and notice what the subject is, in this case they're talking about a movie. You could join in by asking:
Who was in the movie?
What was their favorite part?
When did the movie come out?
Where did they go to see the movie? Did they do anything else fun that night?
Are they interested in [type of movie - syfy, romance, horror, sports]?
To start a conversation with a person think of why you're in the same room in the first place or another subject that you share. For example, if you're waiting for English class to start, you could start a conversation by asking:
What did they think of the homework from last night?
Do they like the book you're reading? Why or why not?
How was their weekend?
How are you today?
What are you watching? (if they're watching a video)
What's up?
Joining a Group Conversation
Joining a Group Conversation
Starting a Conversation
Starting a Conversation