Hello,
Some league hosts (including myself) use/have used "filler accounts" to increae the number of teams in a specific tier to enable promotion/relegation.
This is usually done by creating mutiple accounts with generic email addresses, taking advantage of iGP not requiring email verification when creating accounts and when they aren't needed anymore, you can remove these extra teams and use them again at a later date.
If you wanted to go down a different route, promoting your league in the
Leagues & Events subforum may help to naturally increase the number of managers in your league and enable promotion and relegation.