Maybe when you make a thread, you can allow your first post to be edited by specific users. Cause in some multi user run threads, because of the first poster being unactive, the thread won't work properly

Most people don't pay attention to the first post after there is more than one page...so I think a simpler solution would be to just make a post within the thread that announces it is the new "refer-to" post, and put the summary of what's going on in the thread there and then edit as necessary. Or simply begin a new thread that continues what was going on in the first...you can put a link to the new thread in the old one. =)

Sorry, but the thread I'm working on is pdated very often, therfore linking to a new thread is very inconvinient

Well it's the best suggestion I can give. Adding the ability to give multiple users editing permissions is not something we are able to do, I'm sorry.