Can characters with guard be critically hit?

There have been interactions before where multiple effects stack so that characters can be crit when they should be. This happens because things like guard and crit immunity really just add a large amount of crit avoidance.

Guard from Chewie adds 999,900% crit avoidance. Advantage only adds 200% crit chance, so by itself or even with crit chance on a datacron, it's hard to see it getting through by itself if the Kit cron is working correctly.

Vulnerable reduces crit avoid by 500,000. GAS using Telekinesis against a target with Armor Shred reduces it another 500,000, so if they also had Vulnerable, this is the situation I talked about above where it overrides crit immunity.

The other thing I can think of is someone might have crit chance in their Mastery and it's stacking.

Who was the enemy team?

It was another JMK team. I'm guessing it's advantage that's causing it. There was a lot of damage being done and was watching yellow numbers flying.

What was the full comp, datacron stats and abilities?

Looking at CAT and JMK, they both have crit chance in their Mastery. If it was later in the battle, that's a possible explanation...but it's hard to imagine it ramping up to a million unless something was wrong with the datacron.