Today we would like to announce the launch of our bulk tagging endpoint. Previously, updating tags on many people at the same time meant writing a script that makes lots of API calls. This was inefficient. So now, rather than making individual API calls for each record, you can update them in bulk.
POST /lists/:id/tag/:tag to add the tag to all of the people on a list.
DELETE /lists/:id/tag/:tag to remove the tag from all the people on a list.
More details in the Lists API documentation.