Although OOTP games can have many human managers, only one manager identity can be 'logged in' at any given time. To change the currently logged in manager, use the Select Manager option:

To switch human managers, click on the manager you want to use, and click Select Manager. There is a section of the online manual that talks about OOTP's manager model in detail.

If you are not sure which human manager is logged in, you can look at the Manager menu, or by going to the Manager Home Screen.

Note: If you are playing a game with multiple human managers, remember that if time advances for one manager, it advances for the others as well. Make sure you have made any necessary changes to all managers before proceeding. When you have multiple manager identities in a game, it's a good idea to password protect your manager identities to avoid mistakes. This can be done from the Manager Options page.