Here's the blurb from the wiki.
War Thunder players now have the ability to join in groups up to 4 by using the new Squad feature. The use is simple. Host should go to the top right of the screen where the "Contacts" icon (looks a bit like 2 heads stuck together) which is at the bottom left of this cluster. Host should click on this icon to open another box to the left which will have contacts in it. People whose names are in green are currently available for game play and can therefore be invited to the host's squad. Click on the desired name and then look under this window for a tab with planes on it. This reads "Invite to Squad" when you bring the cursor over it. Clicking this will do just that.
The invited friend should look at incoming messages in the "Chat" icon back in the top right cluster beside the "Contacts" icon. Since the host sent an incoming message, this should have a gold highlight around it.
From here the invited friend should click this chat icon and select the message which mentions accepting the invitation to a squad.
Once this invitation is accepted, the invited friend should go to the bottom of the incoming message then select "Ready" when ready to join the game.
Once "Ready" is selected, the "Chat" window and "Contacts" windows have icons of unavailable friends in red with just a pilot head, available friends in green with a pilot head in green and squad members in gray with a clock or green with a chain link. These squad members in gray with a clock are not yet ready and green with a link are ready.
All squad members can see the readiness status of all other squad members.
Use either the chat feature or Team Speak to coordinate.
To access squad chat during battle enter " /s" (without quotes and press space), or press the "Tab" button and you will be switched to squad chat and your messages will only be seen by your squad instead of whole team.
Once all squad members are ready, the host will select the first "To Battle panel. The game type (Arcade, Historic or Real) then select the server (RU, EU or US) and finally select the second "To Battle" panel to begin the game.
We have experienced some members being left in the hangar; some have gone into different games and some have been placed on opposing games, but this method works better than the old efforts where we all tried to join by pressing the second "To Battle" panel simultaneously.
If one members leaves, then to rejoin, a second invitation must be sent.