Has your project grown? Is it too big for a single user? Do you need to work in an enterprise environment?
When you first start with a project you may just need one person working away at the initial Requirements (single-user environment). However, when more stakeholders, designers, reviewers and managers join (an enterprise environment) the installation needs to grow.
The good news is that in Cradle this is simply a reconfiguration of your Cradle licence for an Enterprise version and then you can all carry on with the same project.