Hi I'm looking for same function. The only way I think to get this is combining roles and jquery.
1) Install a role manager plugin. Create a new role called "Clients". Add news users with this role (if you want to upload logos, you will need to instal User Photo plugin too).
2) Display users with "Client" role via handcoding this function.
3) Make the slider using this jquery plugin
I know that this isn't the best solution, but is the nearest I catch to get what you are looking for. The bad think is that I will can't order users by "relevance" in the future (users can be ordered, by ID, nicename for example).
I also looking for a wordpress plugin to do this easier.
ps: sorry for my english.