Help deciding on which CMS to start with

I may be hiring if this goes off right. So post 'em if you got 'em.

I have read wiki's CMS articles and the Frameworks articles.
as well as looked into the CMSMatrix.

I am still stumped as to which CMS to start off with.
I am worried about security and also expandability down the road or changing the CMS all together if we get big enough.
I'm sure this has been posted before...., but what I need I think I can get from PHPNuke and the modules they offer.

Anyone want to dissuade me?

Or offer another nearly-free tool?
WHAT I NEED:
I need to be able to get users to create groups and not all users need to be able to log in. Just the group's designated admin.
I want to be able to sort the groups by levels and create a schedule of events for them to follow. Like "Group A" must meet "Group B" on this night or whatever. And I want the "Group Admin" to be able to email his entire group from his own admin area.

I want to be able to keep all the data in a DB (duh?!) and I will be running it on an Apache *nix server with PHP 5 on it and MySQL.

I am not DB guy and I do know code just not programming, mostly front-end stuff. This being said - I have no fear.

I think I've said enough.
anyone with some help will be first on the list if we need to hire some people. I promise not to be a drive-by-poster.
This is for real.

http://www.opensourcecms.com/

they have free demos for all the larger CMS's and many of the smaller ones for you to try to see if you like. at the least it will give a list of CMS's for you to try/research on your own

Big thanks.

not a bad resource at all.

Expression Engine is an extremely powerful CMS and is free for non-commercial use, unfortunately i cant post a link as i'm a new member but google should be able to help you out!