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Use the first minute well. Most group discussions give a topic and a short time to think. Write down three points: one for, one against, and one example from something you know. That is enough to take part throughout without repeating yourself.
You do not have to open. Opening is useful only if you have a clear point and can set up the discussion: define the topic, or suggest two sides to consider. A rushed opening with nothing in it is worse than a calm, useful entry a minute later.
When you speak, make one point and stop. 'I think online classes work better for theory than for labs, because labs need equipment most students do not have at home.' One idea, one reason, under thirty seconds. Long speeches lose the group's attention and make it harder for others to respond.
Build on others. 'Building on what she said about cost...' or 'I agree with the point about rural access, and I would add...' This shows you are listening, which is one of the main things the observers are checking. It also makes your point more likely to be taken up.
Getting in when everyone is talking is the hard part. Wait for a breath, then speak clearly and slightly louder, starting with a link to the last speaker. Do not talk over someone mid-sentence. If you keep failing to get in, a short 'Could I add something here?' usually works.
Bring quieter people in. 'We have not heard from you yet. What do you think?' This is one of the most valued moves in a group discussion and few students make it.
Disagree with ideas, not people. 'I see it differently, because...' is fine. 'That is wrong' or a laugh is not. Keep your voice level even if someone else does not.
Near the end, if nobody has done it, offer a short summary: the two or three main views and where the group mostly agreed. Some groups are asked to reach a conclusion. A fair summary is often the clearest sign of someone who can lead.
Practise with friends using a timer and an observer. Ask the observer one question afterwards: 'When did I help the discussion, and when did I get in the way?'