Prime is so silly that it can only read its own .mcdx file format and newer versions of Prime additionally are so inflexible that they can't save in older Prime format. So collaboration isn't possible even if all use Prime (but different versions).
Thats incredible and unbelievably but PTC obviously is not willing or able to do any better.
There is a way to convert legacy sheets in .xmcd, .mcd or .xmcdz format to the Prime format of the version you have installed. The converter is either accessible as a stand-alone program or from within Prime. For the conversion to work you need to have a full installation of the latest Mathcad 15 side by side with your Prime installation.
If you have a license for Prime, the same license file is valid for Mathcad 15, too.
As Prime is a lot less capable and is missing a lot of features compared to real Mathcad, it could be necessary that you have to edit the resulting worksheet a lot to implement workarounds or in the worst case that it does not work at all, if it uses features not implemented in Prime.
Very basic calculation worksheets should convert without much problems, though.
You can find the latest Mathcad 15 M050 here: https://www.ptc.com/-/media/Files/Docs/Mathcad/Mathcad15-EN.zip