If you want to tag a long list of members, please send an Excel file to your Customer Success Manager with all the email addresses of the people to tag and the name of the tag to add.
In Content Metrics > List of posts, you can select all the posts tagged with the same tag and then by clicking on the metric icons, classify them.
You can also retrieve all the tags in Manage > Tags
You can automate tagging for groups in Manage > Add groups > Auto-tagging rules.
The autmoated tagging is based on rules you set according to the name of the group.