A group of people spends weekend in a cottage where they lost one friend ten years ago but they don't know that two of them have been in a secret relationship and it was that accident that had brought them together.

Haven't seen that movie but I certainly will today, thanks.

I see it now, thank you :)

Thanks, I'll work on it.

I'm getting a bit of "The Big Chill" here. I love those movies where a group of old friends are brought together for a weekendand and secrets come out. But I'm not sure if that's what you're trying to get at here.
What brought them to the cottage, the friends 10 year anniversary?
10 years is a long time for a relationship to be kept from friends.

DPG (in all his briefness) is 100% correct. You've given us zero story here. Really what you've provided is the first 15 pages of the script summurised into 41 words.

I mean what is this about? Is it a horror script where the characters start getting picked off one by one. Is it a comedy where hilarious circumstances play out in the cottage? Is it a drama where the relationships of the group are put to the test?

I could go into detail about protags, goals, plot points etc but i'll just say refer to what dpg has written above. Dont give us the set-up to the story - give us the story, or at least an idea of how its going to play out.

Who is protagonist?
What is the protagonist's objective goal?
Who is the antagonist?
What are the stakes?

What is the causal relationship between "where they lost one friend ten years ago" and the "secret relationship" and the "accident that had brought them together"?

What is the dramatic significance of the secret relationship and the accident?