Alert - New scam method

I received an invitation to trade from the user [Removed by Support] Following the usual procedure, I sent an invitation, they approached my hideout, and we made the exchange. After that, I continued on my map and received a notification that the previously mentioned user had left the guild. My surprise was that they were never invited. The user took citadel fragments (among other low-value items).

The question is, how could this user access my guild without having been invited?

GGG needs to urgently address these vulnerability issues.