There is a multitude of ways in which you could format that code:

  app.MapGet(
      "/todos/{id:int}",
      [Authorize("AdminsOnly")](int id)
          => "This endpoint is for admins only");

Or for people who don't like expression bodied members:

  app.MapGet(
      "/todos/{id:int}",
      [Authorize("AdminsOnly")](int id)
      {
          return "This endpoint is for admins only");
      });

Extracted to a method:

  app.MapGet("/todos/{id:int}", GetTodos);

  [Authorize("AdminsOnly"]
  string GetTodos(int id) => "This endpoint is for admins only";

And more...

You could also put these methods in a class and put the attribute on the class, which would solve your "grouping" problem.