The file must be named _group_variables.yml. It inherit from _global_variables.yml, so just update the needed vars.
Save the file in the projects directory or in a subdirectory (to create "groups" of projects)
Create a file named project-name.yml in a group's directory. It inherit from the previous _group_variables.yml, again, update only the needed variables.
The filename is used as project name.
Run ./run.sh projects/group for a full group or ./run.sh projects/group/project.yml for a single project