Projects are a way to collaborate with other users directly and privately within a group. DNA results can be submitted at varying level of detail, allowing collaboration at a granular level that allows for either full disclosure or anonymization as desired. Projects are designed so that you know exactly what data you are sharing, and with whom.
If you are a Gold-Tier DNAGedcom Subscriber you can create projects.
Anybody, regardless of subscription status, can join and participate.