Used Enums for Filtering. Also added a Html extension to build Select Lists from Enum Values

The filtering is now performed using an enum
```
IPagedList<Group> GetGroups(string userId, GroupFilter filter, Page page);
```

The View model maintains the current filter
```
public class GroupsPageViewModel
{
   public IPagedList<GroupsItemViewModel> GroupList { get; set; }
   public GroupFilter Filter { get; set; }
}
```

The dropdown list is built automatically.
```
@Html.EnumDropDownListFor(model => model.Filter, new { @onchange = "LoadGroups();" })
```