How would you address your colleagues at a youth club meeting you will be chairing differently than you would speak to your friends during a tea break about teenage pregnancy in your community?

The core differences in your speech are determined by who your audience is and the genre of the venue. Let's start with audience. The question states that the youth group gathering is colleagues. Because the youth meeting is among colleagues, it sounds more business-related. Your word choice and non-verbal communication should reflect that. You are likely to use jargon, but not slang. Jargon is technical language that your audience will understand. Your body language will also show a more formal attitude. Likely, your body language will be much more subdued than if you were talking to friends. Next is the venue difference. You are chairing a formal business meeting, which means the majority of your remarks will have been planned before the meeting. Your job is to guide things along and keep the meeting on track. Your colleagues will talk, and you have a responsibility to hear them out without interruption. All in all, the way you address your colleagues will be much more formal than when speaking with friends.
The meeting among friends will be very different. You will likely not be able to use jargon, because the terms might draw blank stares. You will, however, be able to use slang, incomplete sentences, and animated body language. The conversation might even be subject to interruptions and teasing. Those two things likely would not happen at the other meeting. Your friends also won't expect anyone to have come to the gathering with prepared remarks.
http://creducation.net/resources/nonverbal_communication/types_of_nonverbal_communication.html

If you address your colleagues at a youth club meeting, you would use more formal language, meaning you would not likely use slang or incomplete sentences. Instead, if you are chairing the meeting, you would likely come up with an agenda for the meeting in advance. You would use full sentences to express your ideas, and you would likely try not to speak extemporaneously (meaning without preparing in advance). At times, you would also invite the members of your meeting to respond to your remarks, and you or someone else might write down their remarks and comment on them. 
If you were speaking about teenage pregnancy to your friends during a tea break, you would likely use informal language, including incomplete sentences and slang. You would not need to prepare your remarks in advance, and your role would be to respond to what your friends say in addition to making your own remarks. A conversation is more about give-and-take than about preparing remarks in advance.