ASP.NET MVC w/ Entity Framework - UpdateException

I am encountering a problem when I go to update an object in my database using Entity Framework.

Firstly, the concept is:

A user can be a member of a group, hence a Group contains Users.

To model this in the database, I have the following structure:

Users: ID Name etc.

Groups: ID Name

GroupMembers: GroupID UserID

Both fields in GroupMembers are foreign keys relating back to the User ID and Group ID. When I load this into Entity Framework, it is modelled correctly in that a Group object has a list of User objects, and each User object has a list of Group objects.

However, when I go to add a User to a Group, I get the following problem:

var group = DAO.GetGroup(GroupID);
var user = DAO.GetUser(UserID);
group.Users.Add(user);
conn.SaveChanges();

Unable to update the EntitySet 'GroupMembers' because it has a DefiningQuery and no element exists in the element to support the current operation.

I have seen various pieces online advising to add an ID field to the GroupMembers table - however if I do that, I get a mapping error in EF that complains about the ID not being mapped to anything.

Would anybody be able to lend some other advice, or a workaround?

Thanks,

Chris