I don't remember exactly, but I think he said that while adding one again, all the others reappeared.
After playing a few more hours in the game, we also had problems after he joined the game. When he pressed Q, nothing happened. Tab also. But after 20 seconds, the popup came. Same for Tab.
There seems to be a huge delay after joining, maybe because the world is synchronized during the first minutes after joining. The lag, when doing something, decreases over time.
We both have good computers and internet connections, we don't experience lags during the game. Only after rejoining a game, my colleague has huge delays for a few minutes, while the world is beeing synchronized.