Unable to Share Outlook Task Folder

I am able to share my task folder with one other office worker. But three others I've tried cannot accept the sharing offer. The button in the sharing message to open/accept the share is grayed out for them. After I send the offer, I can see these individual listed in the Properties-->Permissions tab and confirm they have been given at least Read permissions. I've also made sure that I've only shared the task folder with one individual at a time in case the folder can only be shared with one person at a time (althought that doesn't sound like a reasonable constraint to enforce).



We all have the same setup: Windows 7, Office 2010, using Microsoft Exchange Server. At least one of the three individuals was able to share their task folder with me.



This question was first posted to Outlook.com and was advised to move it to Office.

Moved post to Office and advised to move it to ExchangesrvClients.