How you group your students, like how you support them, is largely dependent on your instructional goals.
You might be interested in allowing each student maximum creativity and individual reflection on the ideas you're mapping. From an assessment standpoint, you also might want to have a detailed view of how each child deals with the concepts.
Small groups allow students to create meaning in groups, which can be a powerful form of learning. Some kids do most of their learning from each other, bouncing ideas off of others. Creating maps in pairs or small groups can be a way to structure the conversations as well as to assess what students' group understandings are.
One of the beauties of the Internet is that students can potentially work in different places and times on the same map. Co-Motion, by Bittco (http://www.bittco.com/) allows you to set up a master map which students can work on wherever and whenever they want. Co-Motion works especially well as a brainstorming tool, allowing students to collaboratively create, comment on, and vote on a set of ideas.
There may be times when you want to use mapping as a group prewriting tool to make sure your students begin from a common set of understandings. Creating a map on an overhead projector at the front of the class before beginning to write individually can be an avenue to doing this. For one computer classrooms, building a basic map in large group on the one computer can be a springboard to creating more detailed maps individually using traditional paper-and-pencil mapping.
Copyright 1999, Jon Margerum-Leys and The University of Michigan.