Create a group and user then assign user to group using Jackrabbit API

CreateGroupAndUserServlet.java

private void createAuthorGroupAndUser(ResourceResolver resolver) {
  try {
    Session session = resolver.adaptTo(Session.class);
    if (session != null && session instanceof JackrabbitSession) {
      // Get our User Manager
      UserManager userManager = ((JackrabbitSession) session).getUserManager();
      ValueFactory valueFactory = session.getValueFactory();

      // Create the Authors group if it doesn't exist already.
      Authorizable authors = userManager.getAuthorizable("authors");
      if (authors == null) {
        authors = userManager.createGroup("authors");
        authors.setProperty("displayName", valueFactory.createValue("Authors"));
      }
      
      // Create the default author if it doesn't already exist.
      Authorizable author = userManager.getAuthorizable("author");
      if(author == null) {
        author = userManager.createUser("author", "letMeIn");
        author.setProperty("displayName", valueFactory.createValue("Default Author"));
      }
      
      // Add author member to authors group
      ((Group) authors).addMember(author);
      
      // Save our session
      session.save();
    }
  } catch (RepositoryException e) {
      LOGGER.error("Could not get the session", e);
  }		
}

API Docs: https://jackrabbit.apache.org/api/2.4/org/apache/jackrabbit/api/security/user/package-summary.html