Why can everyone else add defenders to a gym I'm attacking and I can't? I always get the "this gym is under attack" message.

Yea I'm on the same page as you, I never understood what I do wrong. My friends just put a random Pokemon in it while I'm attacking

If there are any openings when you start attacking a gym, they can be filled. After the first pokemon is kicked from a gym, there is a 10 minute window when new defenders are blocked. If you can't take out all of the remaining defenders during that 10 minutes, then more defenders can be added.

Once a defender is kicked out, a 10 minute lockdown timer is started, during which no one is allowed to fill any open slots. Once the timer expires, all open slots can be filled. If another defender is kicked out after the timer expires, a new 10 min timer is started. I think this was introduced a long time ago to mainly reduce shaving and to a lesser extent allow attackers to have a chance when a gym is actively defended.