Click "Users & Groups" button (bottom middle-
Click on Groups (the picture with multiple heads)
Drag groups (everyone who will be using the program) from here into the ACCESS CONTROL LIST pane.
Set "Permissions" on each to "Read & Write"
Click Save button along the bottom right.
Click on the gear button (the small button just above the Save button) bottom right and select "Propagate permissions".
Make sure "Access Control List" is ticked.