Does adding ram to my mini void the warranty

Not the new ones unless you damage the Mini when you replace them. Some users have posted that they damaged the memory slots when removing/installing the memory.

I have replaced the memory on my 2012 and 2009 mini withno problems

The 2009 and earlier models must be cracked open in order to upgrade the RAM.

The 2010 and later uni-body models have a bottom access cover that makes upgrading the RAM a snap.

Have a look at > Mac mini: How to remove or install memory