Joint activities have been investigated primarily in the context of household-based models of travel demand. The joint decision requires agreement about several issues. Each participant, on the one hand, tries to cooperate with others to reach an agreement on the joint activity and, on the other hand, tries to maximize his or her own benefit. Given such a semicooperative environment, the methods of joint activity scheduling cannot be directly extended. Negotiation is a suitable approach to reach an agreement on multiple issues and decide on a joint activity. This study adopted a multiplayer, multi-issue, multilateral negotiation model by which agents sequentially submitted proposals in consecutive rounds until a deadline. A new credit mechanism for the agents under conditions of incomplete information was proposed. Credit encouraged agents to do favors by compromising in return for agreement on other issues. Some preliminary experimental results show how three agents focusing on four issues related to a joint activity come to an agreement on those issues. The study demonstrates that the number of agreements is improved and that the average number of rounds before an agreement is reached is much smaller than that in a process in which no credit system is used.