types.NewSessionOptions.groupWith

Names a session to group with, not a group: tmux puts the new session in that session's group, or starts one named for it. Members share one window list, so a window created in either appears in both and a window moved in either moves in both — unlike a linked window, where each session keeps its own list and its own index for it.

The group a session ended up in is Session.group. windowName , shellCommand and startDirectory describe a first window, and a grouped session does not make one, so tmux ignores them here.