How would the way you address colleagues at a youth club meeting that you will be chairing differ from how you would speak to your friends during a...

This is simply a matter of adjusting your tone and language choice to cater to your particular audience! Knowing the demographics of your target audience can help you sensitively and effectively convey your argument. 

If you are addressing colleagues at a youth club meeting that you will be chairing, you will want to adopt a formal tone and language to establish your credibility and authority. You will be responsible for leading this team, and will, thus, also be responsible for presenting your ideas and yourself in a trustworthy light. In this situation, you would need to maintain a serious, professional tone, use formal and respectful language, and employ vernacular that is pertinent to your field. You would want to avoid slang, jargon, and humor that could be perceived as impertinent.

If you are talking to your friends during a tea break about teenage pregnancy in your community, you can afford to be much more informal in your tone, language, and approach. Although this is a serious topic, you may adopt a loose, conversational voice. Rather than giving a formal speech, you will find much more success in opening up a dialogue in which everyone participates, shares information, and articulates their opinions. In this instance, you could use slang or jargon as long as it is not inappropriate or disrespectful toward others.