If your account is set up as an Organisation and you have multiple Funnel accounts, you can grant users access to some or all of the associated Funnel accounts.
Open the Funnel account that the user is set up in and navigate to 'Users' in 'Funnel Settings':
Click 'Organisation Users'.
Find the user that you want to edit (you might need to scroll through the pages if there are many users).
Select the user and then click 'Add to accounts'.
This will bring up a modal that'll allow you to chose all or some of the associated accounts:
You can also remove the user from accounts.
When a user has access to more than one Funnel account, they can easily switch between accounts: