The commish invite functionality is awful for sending reminders now...
This was updated, wasn't it? There used to be an option to send a reminder to every team that hasn't joined the league...now it is a painful process to send reminders.
Instead of three clicks to send 120 reminders it now takes 240 clicks to send 120 reminders. This is absolutely stupid.
You can send a mass email to the league.
League page, Email League. There is an All Managers option.