To find the GroupID (used on L4D), you can try:
GroupURL ID - 103582791429521408.
GroupURL ID can be found on the "Enter Chat" link on the group page.
eg:
http://steamcommunity.com/groups/portaltf2 ->
steam://friends/joinchat/103582791430044186
103582791430044186 - 103582791429521408 = 522778
(The "1035827914" will never be changed)
I can't see a functional application to this, but I think VALVe will release more stuff related to the groups...
Edit:
You can access the group page using:
http://steamcommunity.com/gid/GroupURLID
The first group is.... VALVe official group -.-' :
http://steamcommunity.com/gid/103582791429521412