A task cannot be assigned to more than one person. But it can be a group.
You can achieve that by modifying the Task work item type definition (you can export it using witexport.exe). Here you can specify users/groups/values you want to see in the list:
<FIELD refname="System.AssignedTo" name="Assigned To">
...
<ALLOWEDVALUES>
<LISTITEM value="yourdomain\group"/>
<LISTITEM value="yourdomain\user" />
</ALLOWEDVALUES>
...
You can specify as many list items as you want. If you want to refer a project-specific TF group foo, you must define it as [project]\foo (note that you must *NOT* replace the word [project] with the actual project name). For TF server-scoped groups you should use [global]\group form.