“I guess I might as well stick around and have one more.”

You go out to a bar with some coworkers after work on a Friday. You were planning to just have one drink and then leave, but after the first drink you're having fun so you decide to stay. You say:

I guess I might as well stick around and have one more.

I guess (clause)

You use "I guess" to show that you have changed your mind when someone is trying to convince you of something. For example:

A: Why don't you stay and have one more drink?

B: OK, I guess I can have one more.

"I guess" is a phrase which shows that you're making a decision that's different from what you originally were planning on doing. If you simply say "I will ___", it sounds as if you personally wanted to do it:

I'll stick around and have a few more drinks.

In the example at top, the speaker wants to suggest that staying was his coworkers' idea, not his own.

(someone) might as well (do something)

The phrase "might as well ___" is used when there's an action that's not happening yet, but it's almost happening. For example, if you're eating a dessert and you finish 90% of it, there's not much difference between eating 90% and eating 100%. You say:

I might as well go ahead and finish it.

So "might as well ___" is used as a reason for doing something that's almost done already. In the example above, the speaker is saying that he has already had one drink, so there's not much difference between one drink and two drinks.

stick around

"Stick around" means to stay. It is a more casual way to describe staying somewhere.

have (a drink)

You often use the word "have" to describe drinking an alcoholic drink.