Threema is a secure mobile messaging app that puts security and privacy first. It features true end-to-end encryption and you can use Threema completely anonymously, without giving away personal information such as phone numbers or email addresses. Your messages will also be deleted from their servers immediately after being delivered.
The app has just been updated in the Windows Phone store with the ability to create and modify groups. In addition the update has also added minor features such as coloured default avatars and other minor bug fixes and improvements.
The app costs $1.99 unfortunately without a free trial and can be downloaded from the Windows Phone Store here.
Thanks Mahir for the tip.