From The Official Message Board

Concerning the number of player characters:

You can have as many people in your party as there are on the server, as people join your party their portrait will appear on the right side of your screen (if you chose to view them) and if you get more than your screen can hold it will scroll. Also they are putting in an option to just have each PC’s name and condition without the portrait to take up less room if you prefer.

Cord Grimwinder-8d
Interplaygames.com Moderator