When working in a group, you have to be heard to demonstrate your participation. If you have troubles with being assertive and sharing your opinion, start by watching this video about acquiring the right mindset and how to handle your posture and tone.
It discusses that you should start with thinking of what you want to achieve and which words fit this message. You should recognize what you want, but more importantly that the other person has its own point of view. There should be a balance in these two. If you do not take the point of view of the other person into cosideration, it leans towards being dominant. As they say: It's about expressing your own point of view and feeling comfortable you've said what needs saying"
You should choose words deliberately. It is common to use the word "I" (I feel, I want..). This is not an arrogant I, but an I which stores ownership, it expresses what you believe. Posture and tone can influence the message, of which is more displayed in the movie.
A last tip is to rehearse. Skills are not easily to master after watching just one movie. Use these tips to improve your assertiveness skills!