Objective of Spoken Communication
The listener must understand what you are saying.
Before speaking in a GD
Ideation: You have your collection of ideas for discussion on the given topic(ideally 4-8). You have selected the ideas you wish to communicate (atleast 2-4).
Speaking skills required for success in a GD
1. Conciseness and Brevity as required: Since time is a factor in a Group Discussion, you need to inculcate and use the ability to communicate an idea with less words. Write own every idea as they occur to you. Then, revisit the written statement and remove redundancy. Manage ideas as cause - event - effect sub-segments. With enough practice, you will be able to structure concise statements during the ideating process itself.
2. Phrasing speech to your communication strengths and also to the needs of the listener: You can use a non-english language if it helps. Other tools that influence listener understanding are vocabulary complexity, acronyms, examples etc.
3. The volume, tone, diction of speech: Volume has to be enough so that each member of the group can listen. You will need to vary it for different groups or at different times in a same group. The tone of your voice can be varied ranging from a baritone to a nasal sound. You need to practice speech in various tones and settle on the one that delivers power to your speech. Enunciation, clarity of spoken words are factors you need to work on.
Please drop in a comment if you feel any other factors may be an influencer of Spoken Communication in a GD.