From what we have gathered, Hello Games has sent out packages to 16 people, many of which are mods on Reddit. In these packages are a No Man's Sky shirt, pin, socks, and a signed poster that says "hope you like what's next" along with a message saying "dreaming of far-flung worlds."

Each package also contains an audio cassette that are numbered (IE: 1/16, 2/16, etc.). According to a Reddit post, not all tapes have been received yet since most people don't actually still own a cassette player in 2017.

The NMS detective team on Reddit have been hard at work trying to figure out what some of the cryptic audio snippets mean from those that they have received thus far. One such audio clip's spectrograph includes the message of "706s7274616p" which has been decoded.
"Okay, so if you take 706s7274616p and apply ROT13 to it you get 706f7274616c which is a valid hexadecimal value. Converting HEX to ASCII says... You guessed it, 'portal.'

Many of the findings are presented in this Google Doc and this Google Drive folder.