1. Rep signs in
2. His/her Client list populates a grid.
3. On selecting a client row in the grid all accounts for that selected client shows in a grid underneath the client grid.

I am new to DotNetNuke and would like some advice.
This app would be a master\details web page.
Is this possible?

Yes this would be possible with a custom module. I'm pretty sure there are already some modules close to what you want here. Check out the resource directory on dotnetnuke.com and I'm sure you will find sometihng very close if not exact.

hth Bruce