Yes you can share content between the team members but...
Firstly if a project authored in RH11 is opened in RH10, a message is displayed saying that the project can't be opened. That is because projects are not backward compatible. However you can change the <projectname>.xpj file in Notepad. If you look in there for the following line:
<rhpml majorversion="x" minorversion="0">
This specifies the version last RH used. The "x" is the crucial number. I think RH10 is "5" and RH11 is "6" but you can check this. By changing the value and saving the file, you can open a RH11 file in RH10. You'll also need to ensure that you take backups before editing anything and also delete the <projectname>.cpd file.
Plus each time you took the RH10 content you'd have to upgrade the project and check for any issues with it. Then of course, you'd need to repeat the above process to enable them to open the project in RH10.
Secondly if you author using RH11, you must not use any functionality that is not available in RH10. If you do, you are asking for trouble.
If this all sounds like a complicated process that has potential for mistakes, you'd be right. It would be a lot simpler if you could all upgrade to the latest version.
As Column indicates, you can downgrade a project, but it isn't the
easiest way to go. Especially since you may lose some capabilities. You
find downgrade procedures on