What's your goal here? In case it isn't clear, the object this returns is not connected to a PTY. This method is an implementation detail that is used by internal logic that is used to setup a child process.

What do you gain from exposing it outside the crate compared to using std::process::Command directly?

I'm still at experimental phase, but I'm essentially wanting to use our existing implementation to set a process group. Currently that looks something like this:

let command = tokio::process::Command::new();
let std_cmd = command.as_std();
let child = std_cmd.group.spawn()

The .group() implementation is just CommandGroupBuilder::new(std_cmd), so I was hoping to do this:

let pty_system = portable_pty::NativePtySystem::default();
let pair = pty_system.openpty(PtySize::default()).unwrap();
let cmd = portable_pty::CommandBuilder::new();
let group = CommandGroupBuilder::new(cmd.as_command()); // ← new API from this PR
group.spawn();

After typing this up and understanding your reply I see how this won't work, since:

the object this returns is not connected to a PTY

Maybe what needs to happen instead is some kind of option to spawn_command() that can set the group instead? (and then I can remove command-group?

Can you expand on the group thing a bit more?
What are you trying to do?
The default behavior for a new pty is tied up with the use of the setsid(2) system call that happens here:

In turborepo, we spawn child processes for a bunch of scripts (provided by the user) in parallel. We use a process group so we can kill all these process (i.e. forward signals).

I want to keep this behavior. Is establishing as a session leader necessary for this pty? Is it possible to spawn a child process with a connected pty that can be part of a process group set from the outside?

Perhaps what you want is to add a unix specific set_process_group method to CommandBuilder that stores an optional process group id in the builder, then, in the code I pointed to, do something like:

// process_group is pre-extracted from the builder in the same was as controlling_tty
if let Some(pgrp) = process_group {
   if libc::setpgrp(pgrp) == -1 {
      return Err(io::Error::last_os_error());
   }
} else {
  if libc::setsid() == -1 {
      return Err(io::Error::last_os_error());
   }
}