"meet" and "meet up" could be both interchangeable in the sense of seeing and talking with someone familiar you know or someone you don't know at all.
" I met a couple of nice people on my way to Berlin " ( Here is unexpectedly )
" I met him at the x coffee as planned for " ( intentionally)
" Let's meet up for some drinks after work " ( intentionally)
" I met up with her when I left the building " ( unexpectedly )
"Meet up "" could differ slightly to mean coming together in a certain appointed place for a certain aim or to accomplish something as in :
"Let's meet up after work to discuss this matter further ". (an arranged meeting with a purpose ).
"To meet up to " has a totally different meaning and refers to reaching a certain level of required quality .
" He hoped his work meets up to the company's expectation "
Apart from having people as subject , 'meet up ' could be the meeting up of roads or streets as in :
"The 2 streets meet up at the next intersection "
That is basically it ! ;)