Since it is already up and running, has user (and group) integration with phpBB3, and so on, most importantly because I'm bored, I'll write how to get to our Mumble Server.
If you are running Windows or Mac proceed, if you are running Linux find #Linux
First you need to obtain the client from http://mumble.sf.net (in the middle of the page "Get Mumble"), download and install it.
#Linux: try getting mumble from your distro's repository, if you can't find it there, or there is an outdated version (everything older then 1.2.0) try compiling it from source or get a deb/rpm file from an other repository (check the unstable ones from your distribution). If everything fails send me a pm, I'll try to hook you up with a deb/rpm file or the binaries.
Next run Mumble (I assume that you installed software on your system in the past and therefore know how to start a program you just installed) and please take the time to complete the audio wizard that shows up on the first run (you can start the audio wizard again whenever you need it (e.g. new sound, new headset) by selecting "configure"->"Audio Wizard").
Now it should welcome you with a server selection window (if not press crtl+o), select the "Custom Server" tab, click new and insert the following information:
Label: dswp (whatever you want but I think dswp is a good name to use, I use it myself)
Address: dswp.de
Port: 64738 (you may leave this blank as well, it is the default port)
Username: <your_username> (the username you use in the forums)
Password: <your_password> (the password you use in the forums)
Now click "Add" to permanently add this server to your custom servers list.
Doubly clicking the entry (dswp) or pressing "Connect" will bring you on our Mumble server. You are now ready to speak.
Optional: Create a Desktop shortcut (Windows)
Right-click on the desktop, select "New shortcut". In the destination field, insert: "mumble://dswp.de" (without ""). Clicking on that shortcut will connect you to the server and, if specified in the custom servers list, log you in.
Optional: Connecting on startup (Linux, command line)
(prompt)$ mumble mumble://dswp.de