ETM,
There are a few ways to approach this.
You could use the character function action on a drop down object to add all of their names to the activity comment/description each time one is selected.
You could add a limited number of name fields to the Activity table and screen... say 40 names possibly....
If you want that activity to show up under each contact or customer, you would have to include a complex filter in the activity grid on the customer and/or lead screen.
There may be more options than this, but those are the first ones that come to mind.