A group of friends on a camping trip have a chance encounter with an alien species and must keep them from finding out that the planet they intend to destroy is the one that they have accidentally crash landed on.

The phrase "is the one that they" is wordy. Recast the last half, such as: ...must keep them learning that they crash landed on the very planet they set out to destroy.

As for the logline:

1) A group of friends--Doesn't really tell us anything about the lead character, too generic.

2) on a camping trip-- Isn't needed, you can drop this.

3) have a chance encounter with an alien species--Drop "Chance" Isn't needed.

4) and must keep them from finding out that the planet they intend to destroy is the one that they have accidentally crash landed on--This is the meat of your logline. (Also, you don't need to say 'accidentally' crashed)
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"When a lonely science geek and his friends encounter hostile but confused aliens who've crashed landed, the group must deceive the aliens into believing this isn't the planet they're intending to destroy."
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Not sure how they're going to do that, but good luck with this!

You would think the Aliens would have GPS :)

Too many pronouns, it's unclear who exactly is doing what.