We do not currently offer collaboration on the same project from different accounts. However, such an effect may be achieved by using certain workflows, although some care is needed to prevent work being lost.
Two users can be logged in to the same account at the same time. If they are working on different projects then there is no issue, their work won't interfere with each other. However, if they are working on the same project, then it becomes a little more complicated. When you make changes in the Explorer interface, such as grouping topics or adding terms, theses changes are held in the local memory of your browser. When you press the Save&Update/Explore button, the model that is currently in your local memory is sent into the system and applied to your data. This has implications for two users working at the same time, let's say user A and user B. Both users have the project open and are working on the model. User A makes changes to the model and updates the system, however, user B still has the project open and has been building their changes on top of the previous version of the model. When they press save and update, the system's version of the model will be overwritten, without user A's changes. For user B to have user A's changes, they would have to refresh their page after user A has updated. This can cause difficulties with two people working on a project at the same time on the same account. However, if it is carefully controlled that neither person is working at exactly the same time, there should be no problems.
On a related note, as it is possible to share models between each users, collaboration between different accounts can be achieved as follows. Imagine that there are a bunch of users that all have the data uploaded in a project in their account. User A works on a project for a while, but then wants to pass on work to user B. They save their model under an informative name and share it with user B. User B can apply the model to their project and continue developing it. When user B would like to pass on the work back to user A, or even to user C, they can save the model again and share it with another user. When working with several versions of a model, it can be helpful to add a time or date stamp to the name to keep track more easily e.g. Hotels 1st Dec 10:00 (although a time stamp is included on saved models anyway).