[SalesForce] Are lookup fields on the User object (e.g. to Contact) supported by the platform

From my experience, accessing direct group members isn't too bad - you just iterate the group and for all user members just pull their details. If it is a group member, you just recursively call your function that iterates the group.

Where things get really sticky is when you try to figure out the indirect members - I've found that when you start trying to traverse the role hierarchy on a user by user basis, unless you have a flat structure you very quickly run out of SOQL queries.

Here's an example of traversing a group and subgroups to get all user ids:

    public static Set<id> GetUserIdsFromGroup(Id groupId)
    {
        // store the results in a set so we don't get duplicates
        Set<Id> result=new Set<Id>();
        String userType = Schema.SObjectType.User.getKeyPrefix();
        String groupType = Schema.SObjectType.Group.getKeyPrefix();

        // Loop through all group members in a group
        for (GroupMember m : [Select Id, UserOrGroupId From GroupMember Where GroupId = :groupId])
        {
            // If the user or group id is a user
            if (((String)m.UserOrGroupId).startsWith(userType))
            {
                result.add(m.UserOrGroupId);
            }
            // If the user or group id is a group
            // Note: there may be a problem with governor limits if this is called too many times
            else if (((String)m.UserOrGroupId).startsWith(groupType))
            {
                // Call this function again but pass in the group found within this group
                result.addAll(GetUSerIdsFromGroup(m.UserOrGroupId));
            }
        }

        return result;  
    }

As already identified in Thomas Taylors comment it would not be possible to automatically populate manager on a user using the role hierarchy for the following reasons:

1. Manager field on User object only holds one Lookup to another user so if the next level above in the hierarchy has multiple User accounts assigned which one should be selected.
   2. The actual manager could be at a different level of the role hierarchy rather than just the level immediately above.

So it is not any advantage for Salesforce to auto populate when it would often require the Admin to change the field value than for them to have to populate it manually. Delegated admin would also mean that update of this can probably be pushed into the business for larger orgs as well.