Users are only able to accept an invitation to a business account if they:
Do not have their own business account
Have not been added to any other business account
If a user is shown a message like the one below, it means that they either have already been invited to another business account or have their own:
If a user has previously accepted an invitation to another business account, they should contact the Account Owner and ask to be removed from it. Once the user has been removed from that business account, they can be added to another one (by accepting an invitation to it).
If a user has his own account but....
It was created by mistake. The user should retire/delete that Organisation and cancel the account:
The trial expired, the user should retire/delete that Organisation and cancel the account.
It should be transferred to a Partner account, this is how to do it: