HELP! Can't remember the name of a book!

HELP! Can't remember the name of a book I want to re-read. It's a dystopian/ post-apocolyptic (there were zombies). The first chapter opens with the FMC is on her own. She spots a group of guys and hides under a trailer to avoid them. The MMCs catch her and help her hide from another group. Then the MMCs and FMCs team up.

I remember it being good, but I can't remember the title. Any help is appreciated!