I’d first recommend using the Repeater widget for this, as it is the one widget that allows you to dynamically add widgets to the page (think of text bubbles). However, as Repeaters are an advanced feature of Axure RP, I’d encourage you to look through our documentation on the subject before diving in:
And I’d like to mention here that while you can certainly simulate the look and feel of an instant messenger, it isn’t currently possible for two Axure projects to connect/talk with each other, as you would with a live messenger in real time.
With that in mind, here are a couple of example posts that may help you get started with what you’re looking for: