'But if they're be still here' is incorrect. It is perhaps a typo. 'They're' can only be 'they are'. 'They are be' does not exist. ('They were be' does not exist either.)

"They're" is a contraction of "they" and "are." There is no contraction for "there" and "were." Also: Your example isn't a sentence, but rather a dependent clause. And you shouldn't ever use "be" after either "are" or "were."

You can say these things: If they are still here... If they will be still here... If they were still here...

Thanks a lot.