If you belong to more than one project, you should see the corresponding group IDs issuing the command groups on your shell. You are always logged in as your primary project: therefore, if wish to write on the /project folder of a secondary group (e.g. with group ID sXYZ), you will need to change project ID using the following command:
newgrp sXYZ
Then you will be able to write on /project/sXYZ and to check the quota of the secondary project with the usual commands sbucheck and monthly_usage. For further details on the command newgrp, please have a look at man newgrp.