Each airline has its own discussion format.
Pick the format that matches the airline you are applying to and rehearse in real conditions.
An Open Day Group Discussion is the airline interviewer's way of testing one thing: can this candidate collaborate with other crew through a 12-hour flight? Pure English ability matters less than your speaking timing and the courtesy you extend to other speakers.
"I really like what Sarah said about teamwork. To add to that, I think communication is also key — especially when working with passengers from different cultures."
When the discussion winds down, naturally summarize with "So if I summarize what we've discussed..." Interviewers notice the candidate who balances leadership with active listening.