Manage users of several applications by a single admin user

It is possible but not with a public API.

However, a better solutions may be to have only one application where you deploy multiple different projects (a.k. web application) into. Use roles so that only users that are allowed to work on a web application can do so.

If this is not possible, you can try the following:

To access all applications running on a server you can use the following non public API (it may or will change in future without any notice ...):

import java.util.List;
import java.util.concurrent.Callable;

import ch.ivyteam.ivy.application.ApplicationConfigurationManagerNavigationUtil;
import ch.ivyteam.ivy.application.IApplication;
import ch.ivyteam.ivy.application.IApplicationConfigurationManager;
import ch.ivyteam.ivy.environment.Ivy;
import ch.ivyteam.ivy.security.SecurityManagerFactory;


public class AccessToAllApplications 
{
    public List<IApplication> getAllApplications() throws Exception
    {
        return SecurityManagerFactory.getSecurityManager().executeAsSystem(new Callable<List<IApplication>>(){

            @Override
            public List<IApplication> call() throws Exception 
            {
                return getAllApplicationsAsSystem();
            }});
    }

    protected List<IApplication> getAllApplicationsAsSystem() 
    {
        IApplicationConfigurationManager appConfigManager = ApplicationConfigurationManagerNavigationUtil.getApplicationConfigurationManager(Ivy.request().getApplication());
        return appConfigManager.getApplications();
    }
}

The executeAsSystem method is used to turn off security. If you use it then security has to be ensured by the process developer.