My site uses custom role- and mambership providers, and i solved this by modifying the GetRolesForUser method. You could do this in a number of ways. I wanted to check if the user is a member of a specific AD-group, like this:

public override string[] GetRolesForUser(string username)
        {
            DirectoryData userResult = GetUser(username);

            if(userResult == null)
            {
                return new string[0];
                //throw new ProviderException(String.Format("The user {0} does not exist.", username));
            }

            List<string> roleList = GetRolesForUserRecursive(userResult);

            //THIS IS THE MODIFIED PART
            List<string> appendGroups = new List<string>();
            foreach (string thisRole in roleList)
            {
                if(thisRole.Equals("YourADGroup"))
                    appendGroups.Add("CommunityAdmins");
            }
            roleList.AddRange(appendGroups);
            //
           
            return roleList.ToArray();
        }

Thanks, but that did not quite cut it.

http://world.episerver.com/Modules/Forum/Pages/thread.aspx?id=47534

This thread suggests adding a key in EPiserverFramework.config. 
<add name="cmsAdmins" roles="Administrators, WebAdmins, YourAdGroupName"/>

Could I do this for making an AD group a part of CommunityMembers?

It's CMS 6 R1, not 2 btw

I tried, but nothing happend

It seemed like a missmatch in EPi-users vs AD-users.

I removed the EPi-user and started over and it helped when adding a certain AD group to the CMS tree.