You can only play together if both the game version and the mod versions are the same for all players. If you use a version other than the current version, all your fellow players must also switch to the same version.
It's easy to coordinate with friends, so that shouldn't be a problem. But public games are of course more likely to be on the latest version. However, it may well be that public lobbies appear in older versions from time to time (albeit very rarely).

1) Mudrunner не снимали с продаж. Для него даже ДЛЦ с Фордами доступно к покупке.
2) Если ты купил ключ для стима и активировал его, почему стим не обновил игру до последней версии?