Unable to save ParseUser which is not authenticated

Question

Unable to save ParseUser which is not authenticated

I am trying to capture the ParseRelation of a user who has joined a group. I had no problems until I added my Facebook login. Now I get the error "Unable to save ParseUser that is not authenticated" when I try to add a user to the relationship and then save ParseObject. I can join groups created by people who are not logged in with Facebook, and I can join groups that I created (I logged in with FB), but I get an error message with a group created by another user who is logged in on Facebook.

mJoinButton.setOnClickListener(new View.OnClickListener() {@
  Override
  public void onClick(View v) {
    ParseObject group = mGroups.get(getAdapterPosition());
    mJoinedRelation = group.getRelation(ParseConstants.KEY_JOINED_RELATION);
    mJoinedRelation.add(mCurrentUser);
    group.saveInBackground(new SaveCallback() {

      @
      Override
      public void done(ParseException e) {
        if (e == null) {
          Toast.makeText(mContext, "You have joined the group!", Toast.LENGTH_LONG).show();

        } else {
          AlertDialog.Builder builder = new AlertDialog.Builder(mContext)
            .setMessage(e.getMessage())
            .setTitle(R.string.login_error_title)
            .setPositiveButton(android.R.string.ok, null);
          AlertDialog dialog = builder.create();
          dialog.show();
        }
      }
    });
  }
});