When you invite someone to a team there are 2 error messages that may occur:
"There was an error inviting the user, please contact support”
This usually happens if the user you are attempting to invite already has an active subscription.
Please ask the user to check the billing section of their own account, there they should see if they already have an active subscription. If they do and you still want them added to your Team Owner account, please contact our Support Team as they may be able to migrate this license for you. The license would then go into a new team within your Team Owner account."You can't add owners of other teams sorry”
This means that the user you are attempting to invite already own their own team on their account. It therefore wouldn't be possible to invite this user to be a part of your team.
If you are getting the second error message and you are not able to speak with the people you are trying to invite, please contact our support team with the email addresses and they will have a look at the accounts for you to see if those users still require their Team Owner accounts.