Multisite - different theme for each user on one sub-domain installation?

I'm setting up a multisite network of blogs for a big network of consultant groups. Each group will have it's own subdomain on the network, and each group will have around 6 users logging in to their group's Wordpress and posting / authoring content. Are there any plugins available which will let each user choose which theme is used when visitor view their "Author" archive page? It would also need to work on the authors pages, as well as posts. So for example:

http://group1.networkname.com/author/user1

Has a different theme to:

http://group1.networkname.com/author/user2

Anything out there that can do this?