Claire, there can be several reasons why Cassie is urinating when seeing strangers, and I would always advise that you first check that there is no medical reason for her doing this.

It could be that your relationship with Cassie may need restructuring. For example, when a stranger walks into the room they should try ignoring Cassie until she has calmed down, then a soft approach without her becoming too excited. Once she is settled you can stroke her gently and hopefully, this will illiminate the problem.