To create a new group and import targets via a CSV, see our Create Groups and Import Targets article. This article also contains links to our documentation for third-party integrations for this specific case. Furthermore, targets can be imported via XLS and XLSX files.
If the group has already been created, our Importing Targets from CSV article will be a higher quality reference for this specific case.
Groups can also be created via the Integration Store (to get to the Integration Store, navigate to Administration > Integration Store). If groups are created via the Integration Store, then the targets will be imported from the selected integration service when the groups are created.