One feature that can join offline business systems data with online data collected by Google Analytics is the Google Analytics Measurement Protocol. This feature allows businesses to send data from offline sources, such as point-of-sale systems or customer relationship management tools, to Google Analytics.

By using the Measurement Protocol, businesses can link online and offline data in Google Analytics to gain a more complete view of customer behavior and improve their marketing strategies. This feature can also help businesses track conversions from both online and offline channels, and make more informed decisions about their marketing spend.
To use the Measurement Protocol, businesses need to set up a server that can send HTTP requests to the Google Analytics API. They also need to include certain parameters in the request, such as the tracking ID and client ID, to ensure that the data is associated with the correct Google Analytics account and user.
Overall, the Google Analytics Measurement Protocol is a powerful tool for businesses looking to connect their online and offline data and gain a better understanding of their customers. By using this feature, businesses can make more informed decisions about their marketing and sales strategies, and ultimately improve their bottom line.