1) Have you ever experienced issues like this?
As a bossing clan we have. People want to guest in our clan chat to learn bosses, and go bossing with us (getting the benefits of our clan) without actually joining
2) If yes, how did you handle it? If no, what would you recommend?
We tell the guests they can no longer guest after a given amount of time. We allow guests to go with us to bosses as a try out for the clan only. A trial of whether or not we are right for them
3) We've considered disabling guesting and sending a kind yet firm message to these guesters asking them to join or leave, do you think this would be wise?
You have to do what is best for your clan no matter what. These people want to be part of your community without officially being part of your community. It doesnt seem fair to those who actually joined to be part of it.
Computer Science Major