Can I add Tags by activity?

Yes. Tags can be added by activity.

Follow the steps given below to add tags to contacts based on activity:
1. Go to the Settings tab by clicking on the gear icon located on the top right corner.
2. On the left pane, click on Tagging Rules.
3. Click on Add New Rule button on the top right corner.
4. Select Based on activity option in the popup that opens.
5. Choose the appropriate activity condition and the count.
6. Select the Tag that you want to assign to contacts that satisfy the selected activity condition.
7. Click on Add.