Simply, I think there needs to be an in-game messaging system to communicate with those on your friends list. Right now, if two of your friends are playing together, and you want to join them, there is no way to message them asking for an invite. For one reason or another, if you are unable to use an external communications platform, e.g. Discord or Teamspeak3, you’re stuffed. You can’t communicate with friends.
Either that, or allow joining a friend’s squad without an invitation assuming there is space.