In an open letter, the signatories say that Skype users face "persistently unclear and confusing statements" about the confidentiality of conversations on Skype, in particular the access governments and third-parties have to Skype data and conversations.
Their letter - addressed to Skype division president Tony Bates, Microsoft chief privacy officer Brendon Lynch and Microsoft general counsel Brad Smith - calls on Skype to release a regularly updated Transparency report.
The signatories note that Skype has over 600 million users worldwide, effectively making it "one of the world's largest telecommunications companies". It was bought by Microsoft in late 2011 for $8.5 billion (£5.3bn).
They say that many of Skype's users rely on the service for secure communications, such as activists operating in countries with authoritarian regimes; journalists communicating with sensitive sources; or just regular users wanting to make a private call.
"It is unfortunate that these users, and those who advise them on best security practices, work in the face of persistently unclear and confusing statements about the confidentiality of Skype conversations, and in particular the access that governments and other third-parties have to Skype user data and communications," the group say in their letter.
As Microsoft has been integrating Skype into various of software products and services since the acquisition, the group believes that "the time has come for Microsoft to publicly document Skype's security and privacy practices".
They want Skype to regularly release an updated Transparency Report giving data on the release of Skype user information to third-parties, based on the individual country of origin, or the request.
The report should include, they say, "the number of requests made by governments, the type of data requested, the proportion of requests with which it complied - and the basis for rejecting those requests it does not comply with".
According to the group, the report should also contain details of all the user data gathered by Microsoft and Skype themselves, along with information on the data potentially held by third-parties, including network providers or hackers.
But the group also express specific concern over the joint venture of Skype with TOM Online in China, which offers the same Skype functionality to Chinese users as US Skype, but with text filters that block messages deemed "unsuitable for delivery".
"Other companies, such as Google, Twitter and Sonic.net already release transparency reports detailing requests for user data by third-parties twice a year, the group concludes.
"We believe that this data is vital to help us help Skype's most vulnerable users, who rely on your software for the privacy of their communications and, in some cases, their lives."