Sorry but this will be a little long.. I want to show members from picked groups, so If I have following members:

member 1 with groups: group 1, group 2, group 3
member 2 with groups: group 1, group 2
member 3 with groups: group 1

So if I only choose group 1 I will get all members, but If i choose group 1, group 2, group 3 I will only get member 1.

So first ill get all members In all the groups like this

List<List<IMember>> listWithMemberInGroupsList = new List<List<IMember>>();
var memberService = ApplicationContext.Current.Services.MemberService;
foreach (string group in memberGroups)
{
    List<IMember> membersInGroup = memberService.GetMembersByGroup(group).ToList();
    listWithMemberInGroupsList.Add(membersInGroup);
}

Then I got a list with list of members then I have to find the members which is in all the lists, This I made with the intersection metod on a list.

public List<IMember> getMembersToshow(List<List<IMember>> listWithMemberInGroupsList , int i = 0)
{
    List<IMember> MembersToAddToPage = new List<IMember>();
    while (i < listWithMemberInGroupsList.Count)
    {
        int compareListInt = i + 1;
        if (i == 0)
        {
            MembersToAddToPage = groupWithMemberList[i].Intersect(listWithMemberInGroupsList [compareListInt]).ToList();    
        }
        else
        {
            MembersToAddToPage = compareNextListWithLastResult(MembersToAddToPage, groupWithMemberList[compareListInt]);
        }

        i = i + 2; 
    }
    return MembersToAddToPage;
}  


public List<IMember> compareNextListWithLastResult(List<IMember> resultList, List<IMember> compareList)
{
    List<IMember> MembersToAddToPage = new List<IMember>();
    MembersToAddToPage = resultList.Intersect(compareList).ToList();

    return MembersToAddToPage;
} 

Now I cant find a way to make this faster because unfortunately it's a little slow by now.