This is what you want:
public static bool IsUserGroupMember(string userName, string groupName)
{
using (PrincipalContext context = new PrincipalContext(ContextType.Domain))
using (UserPrincipal user = UserPrincipal.FindByIdentity(context, userName))
using (PrincipalSearchResult<Principal> groups = user.GetAuthorizationGroups())
{
return groups.OfType<GroupPrincipal>().Any(g => g.Name.Equals(groupName, StringComparison.OrdinalIgnoreCase));
}
}