Throw in your lot with somebody
Definition 1 of 1
Pro Tip 1/3Choosing Sides
It means deciding to side with someone, especially in a conflict or competition. 
As the two divisions argued, she threw in her lot with the marketing team.
Pro Tip 2/3Shared Outcome
It suggests sharing the risks and outcomes of the group you're joining. 
The villagers threw in their lot with the developers, hoping for a share in the profits.
Pro Tip 3/3Personal Advantage
The phrase often implies a hope to gain personal benefit from the alliance. 
He threw in his lot with the new boss to secure a promotion.