Macro-Enabled Excel file not working when sent to colleagues in .xlsm format

Hi, I have created a relatively basic Excel spreadsheet which contains Macros that were created within the worksheets. All calculations and macros were working perfectly and with some pride, I saved the file. However, Excel requires that the file format is converted to .xlsm (macro enabled). So I naturally 'went with it' and selected the .xlsm extension.  When returning to the file a little later, excel couldn't open the new file!! After a few fiddles with the 'properties', I managed to get the thing open and with a few more fiddles, managed to resolve the 'error messages' that had magically appeared. So all good on MY computer, but when I send the file to collaborating colleagues, they are experiencing difficulties in that the cells containing calculations are not working. I'm sure I will not be the first to have experienced this problem and hope that someone far more intelligent out there may have a solution to this.