Allow specific users edit their posts on specific threads

[t2k7]

Hi again guys! I need your help because i haven't found a solution by searching the forums. My question is how i can permit some specific users to edit their posts in specific threads and even better only their first post. For example a user posts a thread with a list of online shops. And everytime he wants to update, it instead of posting an new post he can edit the first post with the list.

Thank you!