Before beginning to work with GRASS for the first time, you have to create a subdirectory (the GRASS database, where GRASS stores its spatial data) in your home directory. For team work, it is better to create it in a partition /data where everybody in a team has permissions rather than in your home directory.
Within the database, the data are stored in the following directory structure:
grassdata (user/administrator has to create this directory)
For each project a new subdirectory called location is created automatically by GRASS:
grassdata/spearfish (this subdirectory is created by GRASS)
Each user (team member) working on this project has her own mapset:
There is a special mapset called PERMANENT where maps common for the group are stored and protected from overwriting. Team members have access to each other's data but they cannot delete or modify data in a mapset that they do not own.