The reporting system of GymMaster can be used to add charges to any group of members that you can generate a report for. If you only want to contact a specific group of your members then all you have to do is filter out the members who should not receive this charge. For more on how to filter your reports, see the article Filtering your reports
Lets say you want to charge all of your members, a good report for this would be the Current Members report. Navigate to Report & Till > Standard Reports > Member > Current Members > [Show Report], once the report has loaded click the [Charge All] button at the top of the page. This will take you to a selection page where you can exclude certain members or select [Select All]. Now enter the amount to charge, a description for it (Charge Note), and decide when the charge should be collected. Finally click the [Charge] button to confirm.