The word “account” here does not mean a Facebook account. It means an account added to SNAP plugin.
It works like this (in Pro):
You click green “Add new Account” button, you chose Facebook, you set it up and connect to your Facebook account. At the last step you chose profile, page or group.
You click green “Add new Account” button again, you chose Facebook, you set it up and connect to the same or different Facebook account. At the last step you choose a different profile, page or group.
Now you got two accounts where SNAP will autopost.
If you need to setup 20 groups from the same Facebook account, there is a “Duplicate” button. You just need to click it 20 times and then select a different group for each account copy.
Free version can post to only one profile, page or group.