Transfer function design in volume visualization has been a challenging problem due to the huge design space. In this work, we present a system which is capable of integrating the transfer function design results from a group of users. For a specified volume dataset, intermediate and final transfer function designs for many users with different backgrounds are collected. A 2D representation of the transfer function feature space, called transfer function map, is then constructed for each volume data set by MDS projection of the collected transfer function samples. With the proposed transfer function map, interactions, including flexible navigation in the transfer function feature space and transfer function design recommendation, have been developed.