Are you experiencing issues involving:
1. Being disconnected during/near the beginning of missions
2. Losing connection to the host, especially during the initial loading screen
3. Losing connection to other players while in game
4. Able to play solo missions, but not join/play with others
Many of these issues can be caused by the same underlying problems. Most often it is caused by a CPU/GPU that is not strong enough to properly run Warframe in a group environment. If you are running into these issues, please check your Graphics Display Unit in Windows and verify that it is above the minimum specs for the game. If you are using Intel's integrated graphics (HD 2xxx, HD 3xxx, HD 4xxx, HD 5xxx) then you may need to turn down some/all of your graphics settings, disable DirectX 11, and change your computer type in the Warframe settings to Laptop.