code: private void addUserToGroup(@Nonnull String providerUserId, @Nonnull String providerGroupId) throws CloudException, InternalException {
          if( logger.isTraceEnabled() ) {
              logger.trace("ENTER: " + IAM.class.getName() + ".addUserToGroup(" + providerUserId + "," + providerGroupId + ")");
          }
          try {
              CloudUser user = getUser(providerUserId);
              if( user == null ) {
                  throw new CloudException("No such user: " + providerUserId);
              }
              
              CloudGroup group = getGroup(providerGroupId);
              if( group == null ) {
                  throw new CloudException("No such group: " + providerGroupId);
              }
  
              Map<String,String> parameters = new HashMap<>();
              parameters.put("GroupName", group.getName());
              parameters.put("UserName", user.getUserName());
              try {
                  if( logger.isInfoEnabled() ) {
                      logger.info("Adding " + providerUserId + " to " + providerGroupId + "...");
                  }
                  invoke(IAMMethod.ADD_USER_TO_GROUP, parameters);
                  if( logger.isInfoEnabled() ) {
                      logger.info("Added.");
                  }
              }
              catch( EC2Exception e ) {
                  logger.error(e.getSummary());
                  throw new CloudException(e);
              }
          }
          finally {
              if( logger.isTraceEnabled() ) {
                  logger.trace("EXIT: " + IAM.class.getName() + ".addUserToGroup()");
              }
          }
      }
docs: Executes the function of adding a user to a specific group as AWS does not support bulk adding of a user to a group.
  @param providerUserId the user to be added
  @param providerGroupId the group to which the user will be added
  @throws CloudException an error occurred in the cloud provider adding this user to the specified group
  @throws InternalException an error occurred within Dasein Cloud adding the user to the group