Consider the following two scenarios:

//Data Contract
public class MyValue
{
}

Scenario 1: Using a static helper class.

public class Broker
{
    private string[] _userRoles;

    public Broker(string[] userRoles)
    {
        this._userRoles = userRoles;
    }

    public MyValue[] GetValues()
    {
        return BrokerHelper.GetValues(this._userRoles);
    }
}

static class BrokerHelper
{
    static Dictionary<string, MyValue> _values = new Dictionary<string, MyValue>();
    public static MyValue[] GetValues(string[] rolesAllowed)
    {
        return FilterForRoles(_values, rolesAllowed);
    }
}

Scenario 2: Using an instance class.

public class Broker
{
    private BrokerService _service;

    public Broker(params string[] userRoles)
    {
        this._service = new BrokerService(userRoles);
    }

    public MyValue[] GetValues()
    {
        return _service.GetValues();
    }
}

class BrokerService
{
    private Dictionary<string, MyValue> _values;
    private string[] _userRoles;

    public BrokerService(string[] userRoles)
    {
        this._userRoles = userRoles;
        this._values = new Dictionary<string, MyValue>();
    }

    public MyValue[] GetValues()
    {
        return FilterForRoles(_values, _userRoles);
    }
}

Which of the [Broker] scenarios will scale best if used in a web environment with about 100 different roles and over a thousand users.

NOTE: Feel free to sugest any alternative approach.