I need to find out which GroupID's refer to this particular group link
The list of ID's of a user's groups I get through this request: link
How could I do this check in PHP?
Only add "memberslistxml /? xml = 1" at the end of the URL.
This causes the page to return an XML that you can "pull" with cURL and XMLSimpleObject.
The full link would be:
http://steamcommunity.com/groups/NewSteamCommunityBeta/memberslistxml/?xml=1
@EDIT
There are some tags on the group page that show the id of this group:
You can try to access it from file_get_contents ('urldogrupo'); and then with a preg_match (); capture this information.