Sometimes you may get the following error when attempting to add a person to your Salesforce instance:
This error is related to your Salesforce's API usage limits.
Your instance limits are shared among all of the apps you have connected to Salesforce. If you get this error, the first thing to do is check if another system is heavily using it.
To increase your credits, you can either add more Salesforce users (each user gets another amount of API calls per day) or buy more API credits.
Finally, Salesforce measures API calls on a trailing-24-hour-window basis, so sometimes just waiting a few hours can free up your API limits.