code: public void sendGroupContact(final String groupId, final String[] contactIds) throws NotFoundException, GeneralException, UnauthorizedException {
          // reuestByID appends the "ID" to the base path, so this workaround
          // lets us add a query string.
          String path = String.format("%s%s?%s", groupId, CONTACTPATH, getQueryString(contactIds));
          messageBirdService.requestByID(GROUPPATH, path, null);
      }
docs: Adds contact to group. You need to supply the IDs of the group and
  contact.