Hey! Sorry that it has taken me so long. I dont' often come to the message board.
I am using eGroupware. It is working well at this point, and interfacing with MySQL and Dreamhost just fine. It wasn't the easiest thing to get running.
I had to compile PHP with IMAP and GD and some other things. There was plenty help around on the web, but perhaps dreamhost has already compiled PHP with those things, as most of us run it as CGI now anyway.
So, I'd start with eGroupWare before I would try PHP groupware. I've tried them both, and believe (personally) that the eGroupWare program is more polished and secure. I've also found the group that runs and works on the code at SourceForge to be great to work with.
If you are a company with money, and want to just get it up and running fast, they do have a installation help for low $$$. If you are interested...
Swing by SorceForge...