For a completely /different/ idea... I know several nonprofit and not-for-profit groups who coordinate their email and activities using a combination of GMail, google calendar(s) for scheduling, google apps for shared documents, and google group(s) for message board functionality. You can pull all this together with a google website, and put it under a domain name. The advantages of doing it this way are no costs at all, no hardware, no hardware maintenance. you just have to figure out how to put the google pieces together and teach your users how to use this mashup you've created from the various google pieces... the downside is that google is running everything, and you're relying on google goodwill to continue running this. your files and data are stored on their systems so you're subject to their privacy policies. and, of course, if your internet link is down, you'll have no access to any of it. otherwise, um, if you really do want self hosting... pick your favorite email server (postfix, sendmail, etc), use cyrus imap, let your clients use any imap email app they prefer (Mozilla Thunderbird, Microsoft Outlook or Live Mail, etc) and use openLDAP for a directory service. A wiki like DokuWiki can provide for group shared stuff... add in a calender server (webDAV wth iCAL files, or similar) that can be used with Thunderbird's Lightning Calendering plugin, and you've got quite a bit of groupware functionality right off the bat, including meeting invitations.