Mumble is awesome, here's a quick guide to getting it setup:
(TL;DR Positional audio on, set up hotkey whisper to team, type !mumble in game chat to connect to your team)

1: Download http://sourceforge.net/projects/mumble/files%2FMumble%2F1.2.1%2FMumble-1.2.1.exe/download
2: Install it (you just need the client)
3: Start it and click "Yes" to configure sound
5: Choose the right devices and check positional audio (This is so cool - you hear where people are in game)
6: The next few screens are self explanatory
7: Once you get to the server connect screen add the server veryhappythings.co.uk port 8767

So we get the tactial of team chat but the sociable of all talk set up a key to talk to just your team:
8: Click settings-> shortcuts -> add
9: double click unassigned and choose whisper
10: double click <empty> and the ...
11: select "whisper to channel", "current" (turn off whisper to linked channels)
12: Pick a key (caps lock works well)
13: Done!

When you are in TF2 press "y" to open up game chat and type "!mumble" this will connect you to your team automatically!

By default you will talk to all but when pressing the hotkey you will just talk to your team

It's zombie punching time!

Hi guys! Yesterday was a tiptop A1 clubbing jam fair. We decided that next week would be a League of Legends night, so if you haven't, download it at www.lol-europe.com and have a go. It's like DOTA, but new. And it's free.

We meet up on Tuesday at about 7:30pm! We usually use skype to communicate, so get that. If you don't have people's skype usernames, hassle us on steam.

We now have 4 servers up - each with a different game mode - as the offical servers are a bit laggy I'm getting awesome 32ms ping!

They are hosted on Amazon EC2 so won't be up 24/7. If you want me to bring them up email ben.jeffery@gmail.com. Enjoy!