We need dxdiag and files from this folder for solve your problem. C:\Users\<Name>\AppData\Local\Arma 3\
Can you upload somewhere in winrar package please?
When package will be smaller than 2,097k, so you can attach here. When package will be bigger, please use some free sharing service and post link here. Thank you.

http://feedback.arma3.com/how-to-user.html.html

Exact issue happens to me and some of my other friends. The only workaround we've come across is to start a MP mission with just 2 people and have anyone else join in after those two are in game.

I looked in C:\Users\<Name>\ but I don't have an AppData folder
Also these crashes can be avoided if I launch the mission then tell my friends to join.

Easy way to get to your AppData. Open File explore and in the address type in %Appdata%. (Percent sign) Appdata (Percent sign again). It will bring you to your Appdata Roaming. This one folder ahead click on Appdata in the address bar and then you will see your local data folder in Appdata. Then you want to Winrar or 7-zip that folder to a smaller and easier package to transfer.

Mass closing ancient tickets with no activity this year; assume fixed or too trivial etc.

If this issue is still relevant in current dev build, please create a new ticket referencing this one and request for it to be re-opened.