Fortieths get organised across four separate chats
At twenty-five you have one group of friends. At forty you have several that have never met each other: the school ones, two sets of colleagues, the running group, the neighbours, the family thread, and three people from university who mostly communicate in photographs of their children.
Every one of those chats needs the same invitation, and the usual approach is to type it out four times with four subtly different sentences. Then the answers arrive in four places, and the total is a number you believe but cannot prove. Two people think they are bringing a partner, three people think they are not being invited.
The fix is to write the plan once, on a page, and send the same link into every chat. It is not a grander way of doing things; it is a smaller amount of work, and the only way to know how many people are actually arriving.