Joining in progress isn’t supported “at this time”, as per Crystal Dynamics.
As the days count down to the launch of Marvel’s Avengers, Crystal Dynamics has been sharing more details on what to expect. It confirmed that there would be over 80 War Zones and Drop Zones at launch. In a recent blog post, it provided more details on co-op, including how the game would match players based on Power Level.
Some interesting points to note though. When matchmaking with other players, pausing the game isn’t possible (naturally). However, it also confirmed that joining in progress wasn’t currently supported. If you drop out of a particularly challenging mission and don’t receive your completion rewards, then tough luck it seems.
The usage of “at this time” seems to indicate that Crystal Dynamics could add it later. We’ll have to wait for more details on that front. Crossplay also isn’t supported “at this time” but at least you can mute other players. Marvel’s Avengers is out on September 4th for Xbox One, PS4, PC and Google Stadia. Servers and the day one patch go live on September 1st.