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 selected six records from the search results for whom I want to add my Activity.
My next step would be to click Contact Options and then select Bulk Actions:
On the resulting modal I would then select Add Activity from the list of Actions, at which point I'd see the Add Activity modal:
After entering the Activity information and creating 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.