First you will need to create the filter or group which specifies the contacts to which you want to assign the activity. Go to Tools > Filters & Groups and create your filter or group.
To then schedule an activity to that group, begin by selecting Schedule. Choose the type of activity.
In the next window you will find the Filter/Group tab. Check the box labeled ‘Schedule to a group of contacts’ and use the drop-down list to choose the appropriate filter or group. (You may also choose ‘ALL Contact Records’.) A Pending activity will be posted to each contact record, and the activities will appear as usual in your Activities and Calendar views.