How do you return the user Id with a generic repository method in entity framework?

I want to return the key field by doing something like this:

public virtual int InsertWithReturnId(TEntity entity)
        {

            dbSet.Add(entity);
            return entity.Id;
        }

Many thanks

EDIT #1:

I dont need a save changes... it's done like this:

    public GenericRepository<Group> GroupRepository
            {
                get
                {

                    if (this.groupRepository == null)
                    {
                        this.groupRepository = new GenericRepository<Group>(context);
                    }
                    return groupRepository;
                }
            }

My pulling (stores successfully the group)

public class GroupMethods
    {
        private UnitOfWork uow;
        public GroupMethods() { this.uow = new UnitOfWork(); }
    public int createGroup(string groupName)
            {
                Debug.WriteLine(groupName);
                Debug.WriteLine(HttpContext.Current.User.Identity.GetUserName());
                ApplicationUser user = ClanMethods.getUser(HttpContext.Current.User.Identity.GetUserName());

                if(groupName != "" && user != null)
                {
                    Debug.WriteLine("inside if");
                    Group grp = new Group();
                    grp.creator = user;
                    grp.Members.Add(user);
                    grp.GroupName = groupName;
                    var id = uow.GroupRepository.InsertWithReturnId(grp);
                    uow.Save();
                    Debug.WriteLine("id:"+id);

                    Debug.WriteLine("stops here");
                    Debug.WriteLine("grp id:"+grp.GroupId);
                    return id;
                }
                return 0;
            }
}

I was hoping I could do grp.GroupId but it keeps failing, so I guess not. Will update on interface in 20 mins

EDIT #3

I am getting:

Warning 1   Type parameter 'TEntity' has the same name as the type parameter from outer type 'gdfinal.DAL.GenericRepository<TEntity>'
Error   2   The best overloaded method match for 'System.Data.Entity.DbSet<TEntity>.Add(TEntity)' has some invalid arguments
Error   3   Argument 1: cannot convert from 'TEntity [....\DAL\GenericRepository.cs(16)]' to 'TEntity'  
Error   4   The type '...chatmodels.Group' cannot be used as type parameter 'TEntity' in the generic type or method '....GenericRepository<TEntity>.InsertWithReturnId<TEntity>(TEntity)'. There is no implicit reference conversion from '....Models.chatmodels.Group' to 'gdfinal.DAL.IIdentifier'.   

when using

public virtual int InsertWithReturnId<TEntity>(TEntity entity) where TEntity : IIdentifier
        {
            dbSet.Add(entity);
            return entity.Id;
        }

It is successfully inheriting the Interface but really doesnt like it :(