Paging Congressman Billybob....
Any interested organization -- and there can be 200 or more of such -- can file a friend of the Court brief. (That is usually my part in Supreme Court cases.)
An indication of how important a brief filed by the government is, is the occasional case when the Court reaches out and asks the government to brief an issue. When such a request is made, though not compelled to do so, the Solicitor General's Office always, to the best of my memory, promptly complies.
So, a brief filed by the government is not controlling or dispositive, but it is more important and more carefully considered than any other single brief filed in any case.
Hope that answers your question.
Congressman Billybob