There are two ways to counter an armbar from the guard: one is to stack the person (bend them in half then pull your arm out), and the other is to pull the arm out.
This video shows two counters to the stack. Many students don’t go for armbars from the guard because they are afraid of getting stacked or getting their guard passed. These counters will help you go for armbars because you have a plan for their counter.
If you are able to execute this first counter in live rolling in front of your instructor, you will be immediately promoted to black belt! Just kidding. It’s a great technique though.