After Effects project files are not OS dependent. Most likely the message they are getting is a notification that they are using a different version of After Effects than you are. This message includes which version of After Effects, and on which OS, the file was created.
So the question you should be asking is: what version of After Effects do you have and what version do your friends have?
If your friends have a newer version of After Effects than you do, all they need to do is click OK and the project file will open.
If your friends have an older version of After Effects than you do, the situation is slightly more difficult. After Effects projects are not backwards compatible. But you can use the File > Save As > Save a Copy As CC (or CS6, CS5.5, etc.) to save one version back. If your friends are using a version of After Effects that is more than one version older, you'll need to do this progressively through multiple older versions.
Since you are making a template, note that when you save as a previous version, any functionality that is new in a given version will be stripped out when you save it as a previous version. For example, compositing options and masks for effects are new in After Effects CC 2014 (13.0) and won't be present in a copy of the file saved as After Effects CC (12.0).
Note that if your project uses source footage files, your friends may need to use the Replace Footage command in After Effects to locate the files, since file paths are different on Mac OS than they are on Windows. Tip: using the Collect Files command before you send the project will bypass this problem.
If your friends continue to have issues with your project file, please post more details to this forum, or better yet have them post here.
Makes sense?