Sending an invite
Open Steam
Start ARMA 3 Beta
Join a multiplayer server
Shift + Tab to open Steam overlay
Invite friend from friends list
Accepting an invite
Have a friend follow the steps above, but invite you to a multiplayer server.
Joining a friends game
Have a friend join a multiplayer server
Shift + Tab to open Steam overlay
Right click friend playing ARMA 3 Beta
Select Join Game