You can learn more about working with Activities in Redtail here. In this post I just want to cover how you would go about adding identical activities for multiple contacts at one time.
From your Advanced Search page, after running a search that will include your desired contacts, place checks to the left of the names of those contacts to whom you want to add the Activity:
In the example above, I've run a search that yielded 99 records (only partial results are displayed in the screenshot) — I then selected six of those records for whom I want to add my Activity.
My next step would be to click Options in the Result Options widget to the right of my search results and then to select Bulk Activity Entry from that menu:
After clicking Bulk Activity Entry, I'll then see the Add Bulk Activity Entry dialog, where I can add my Activity, with all relevant details, and then click Save Activity:
After saving the Activity, each of these six contacts will have identical activities recorded within their record and on the Calendar.
As you can probably see, it's a quick and easy method to add activities to multiple records at one time, when the need arises to do so.