var query =
            from foo in fooStuff.AsEnumerable()
            group foo by foo.Field<Int64>("FooID") into g
            select new
            {
                    FooID = g.Key,
                    FooTier = g.Min(foo => foo.Field<int>("Tier"))
            };

var query =
        from foo in fooStuff
        group foo by foo.FooID into g
        select new
        {
                FooID = g.Key,
                FooTier = g.Min(foo => foo.Tier)
        };

var fooList = fooStuff.AsEnumerable().ToList();

var list2 =
(from foo in fooStuff.AsEnumerable()
select new {
    FooID = foo.Field<Int64>("FooID")
    Tier  = foo.Field<int>("Tier")
}).ToList();

var query =
from foo in list2
group foo by foo.FooID into g
select new
{
        FooID = g.Key,
        FooTier = g.Min(foo => foo.Tier)
};
var results = query.ToList();

public static Dictionary<TKey, List<TSrc>> TestGroupBy<TSrc, TKey>
     (this IEnumerable<TSrc> src, Func<TSrc,TKey> groupFunc)
    {
        var dict= new Dictionary<TKey, List<TSrc>>();

        foreach (TSrc s in src)
        {
            TKey key = groupFunc(s);
            List<TSrc> list ;

            if (!dict.TryGetValue(key, out list))
            {
                list = new List<TSrc>();
                dict.Add(key, list);
            }
            list.Add(s);
            }

        return dict;
}

var results = list2.TestGroupBy(r=>r.FooID)
      .Select(r=>  new { FooID = r.Key, FooTier = r.Value.Min(r1=>r1.Tier)} );