Ah, prepositions and phrasal verbs... They must be so fun for learners. To complicate matters, and perhaps this a feature of British English, but the question 'are you up to it?' has an additional meaning. It means 'are you capable of doing it?'

Example situation. Someone has to give an important presentation but has been feeling unwell in the days leading up to it. Their boss might say to them:

It's important this presentation goes well. Do you feel up to it or should someone else step in? 

It means 'do you feel that you are able to give the presentation' in this situation? More generally, do you feel able or capable of doing something?

I'm a native English speaker... Canadian.  In your sentence context "We are going to have some drinks... are you up for it?"  To be "up to" something is a phrase that asks the question "what are you doing?"  What are you up to (what are you doing now or later today as a general question).

An example dialogue:

Q:  What are you up to?

A:  Nothin'...

Q:  We're going to have some drinks... are you up for it?  Usually shortened "you up for it."

A:  Ya, I'm up for it !  or Ya, I'm in ! 

If you are 'up for' something it means you want to do it.

Are you up for going to a movie?

Sure, I'm up for it!

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'Up to' means what you are you doing.

What are you up to?

I'm practicing English.

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'Up to', used another way, can mean something is your choice.

What do you want to do?

It's up to you.