While this isn’t something we do in Balsamiq Cloud yet, we are working on an awesome solution (you can read more about it here if you’d like). We don’t have a timetable for it as we have just started designing it, but we know it was important to folks, and we wanted to make sure to tell folks that we were working on it.
In the meantime, if you want to share a single mockup with someone, you can always send them an exported png. It’s a perfect workaround, and I’m sorry about that, but hopefully it’s good enough until we have something better.
I’m sorry I don’t have a better answer for you right now, but we totally hear you. This is something that a lot of folks are looking for, and we want to make sure we deliver an awesome solution.