If you source the project source file, you should be up and running with correct umask and group settings. However, if you start copying files back and forth into the project area with your standard umask and group setting (i.e. not sourcing the source file) you will lock the files for your colleagues.
If your group experiences issues with this, please use the chgrp and chmod commands, for example:
chmod -R gu+rwx *
chgrp -R projektgrupp1X *