After you've created an account by logging into the game for the first time, go to portal.playhighguard.com.

Log in to the platform associated with your primary Highguard account. 

This will be the account that all progress is shared from, so if you've made progress on two different platforms already, the progress on the other account will be lost

.Now that you've logged in on you primary platform, select another platform to sign in with and those accounts will be linked. 

Progress and unlocks will now be shared between those accounts. 

Wallet balance will be the only thing that remains separate.