When the helicopters spawn, then it is not possible to join the helicopter if someone else is flying - even though the 2nd seat is available.
It only works if you spawn on a squad mate directly. On BF3 this was at least possible.
Sadly same case for previous BF and they never changed it... so only premades can basically make most of it.
This is a feature, not a bug. It makes it so that you can actually get your squadmates/friends in the heli, instead of randoms. This allows you to use voice comms and teamwork. In BF 4 and earlier, it was almost impossible to get your squadmate in the heli with you because randoms would instantly spawn in it. That was incredibly frustrating. I'm glad they changed it.
If you want to fly with someone who's not in your squad, you can ask them in team chat. Sometimes they will pick you up.
