First, your request URI is incorrect. For creating a group using version 0.8 of the Graph API, it should be in this format:
https://graph.windows.net/yourtenantdomainname.com/Group
Your tenant might also be an *.onmicrosoft.com address. A couple other things: the service currently doesn't support setting the DirSyncEnabled or Mail properties. They are both read-only. And currently you need to have MailEnabled set to false and SecurityEnabled set to true.
To create a group using 0.8 version, this is what your request will look like:
POST https://graph.windows.net/yourtenantname.com/Groups HTTP/1.1
Authorization: Bearer eyJ0eXAiOiJK...vYiFqfkg
Host: graph.windows.net
Content-Type: application/atom+xml
x-ms-dirapi-data-contract-version: 0.8
Content-Length: 725
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<entry xmlns="http://www.w3.org/2005/Atom" xmlns:d="http://schemas.microsoft.com/ado/2007/08/dataservices" xmlns:m="http://schemas.microsoft.com/ado/2007/08/dataservices/metadata">
<content type="application/xml">
<m:properties>
<d:DisplayName>testingGroup</d:DisplayName>
<d:Description>Test group</d:Description>
<d:MailEnabled>false</d:MailEnabled>
<d:ObjectType>Group</d:ObjectType>
<d:SecurityEnabled>true</d:SecurityEnabled>
<d:MailNickname>firstGroup</d:MailNickname>
</m:properties>
</content>
</entry>
Note that 0.9 of the Graph API was released recently: http://blogs.msdn.com/b/aadgraphteam/archive/2013/03/03/0-9-version-of-azure-active-directory-graph-now-available.aspx
If you want to create a group using the latest version of the API, this is how your request would look using XML as the payload (note the camelCase properties and "group" in the URI):
POST https://graph.windows.net/yourtenantname.com/groups?api-version=0.9 HTTP/1.1
Authorization: Bearer eyJ0eXAi...YiFqfkg
Host: graph.windows.net
Content-Type: application/atom+xml
Content-Length: 627
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<entry xmlns="http://www.w3.org/2005/Atom" xmlns:d="http://schemas.microsoft.com/ado/2007/08/dataservices" xmlns:m="http://schemas.microsoft.com/ado/2007/08/dataservices/metadata">
<content type="application/xml">
<m:properties>
<d:displayName>testingGroup</d:displayName>
<d:description>Test group</d:description>
<d:mailEnabled>false</d:mailEnabled>
<d:objectType>Group</d:objectType>
<d:securityEnabled>true</d:securityEnabled>
<d:mailNickname>firstGroup</d:mailNickname>
</m:properties>
</content>
</entry>
And finally just for fun, if you want to use the new supported minimal JSON the payload would look like this:
{
"displayName": "testingGroup",
"description": "Test group",
"mailNickname": "firstGroup",
"mailEnabled": false,
"securityEnabled": true
}