You could use the FieldCompareSetPredicate, should look like:
RelationPredicateBucket bucket = new RelationPredicateBucket();
bucket.PredicateExpression.AddWithAnd(new FieldCompareSetPredicate(GroupFields.GroupId,
GroupItemFields.GroupId, SetOperator.In, (GroupFields.GroupId == GroupItemFields.GroupId), false));
Find at the Docs or Forums for FieldCompareSetPredicate for more info