Can I add users inside my Trello (and pay for them, obviously) or do they have to open their own acc

[Trello Camco]

I’m brand new to Trello and unclear on this: Can I add users inside my Trello (and pay for them, obviously) or do they have to open their own account and then get invited by me? 

[Victor N]

Hi there, 

You can invite people to your Trello workspace either before or after their Trello account creation. Either way, they will still need to sign up for a Trello account. 

I will suggest you invite them to your workspace by following the steps below and then mention to them to sign up for an account to access the boards, that's for those who do not already have a Trello account:
https://support.atlassian.com/trello/docs/inviting-people-to-a-workspace/ 

[Trello Camco]

Thank you.