Hello,
I am not sure if tmuxinator can handle group sessions. So I decided to open this issue.
From my understanding, tmuxinator comes short in automating something like this:

$ tmux new-session -s main
$ In "main", apply rules and layouts defined in main.yml

Now, in another terminal, connect to that existing window group:

$ tmux new-session -t main -s child
$ In "child", create a new window and apply some rules and layout.

I know we can use on_project_exit and attach: false field in the yml file but I could not reproduce the above behaviour with them.