Here's the situation:

I've got an XLSM workbook in a shared network folder which is a trusted location.
It's a shared workbook used by up to 7 different users at any given time; Office versions include 2007, 2010, and 2013.
The workbook is slow to open, save, close, and run the macro. Occasionally, this results in "Not Responding".
Every now and then, somebody will receive a notification that the file is locked by another user, even though they just saved some edits a few minutes prior.

With all of this happening, I'm looking for a resolution to make things faster and easier. I've tried researching for some resolutions, but I'm not sure which is best suited for my situation. I've seen one possibility mention having multiple workbooks feeding into a master workbook, which I'm not to sure how to do since the master would be the shared macro workbook. Additionally, we're using this workbook to keep track of prospect progress, so it would be easier if we only have the one workbook to worry about.

Another option I saw mentioned creating a document worksite, which would allow multiple users to make edits and see real-time updates, and this would all sync with a localized master file. This seems to be the best option so far, and it seems that SharePoint would be the way to go. Unfortunately, I'm not sure which SharePoint program would work for us, considering we have users with varying Office versions. I was originally looking at Windows SharePoint Services 3.0 with Service Pack 2, but it does not support the Windows 2011 Server we're using. SharePoint 2013 looks like it only works with Office 2013, which only two users have installed.

What would you recommend? SharePoint seems like the way to go, but I have no idea which one to go with. Otherwise, is it possible to create an online document worksite which would allow multiple users to make edits which would sync with a local file? Any help/insight would be greatly appreciated!

Thanks!

Eric