To get up with someone means to contact them, to be in communication.
I got up with Mary after you left and told her all about you.
To go up with someone usually means to accompany them.
Can I go up with you to the mall?
In these cases, the word [I]up[I] is not functioning as a preposition, but as a particle whose placement in the sentence is more or less superfluous.
Other examples are: Meet up, and start up
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