How to create a New Activity group in ACS People

First go into group setup in people,  at the top as group type pick activity
then click add and add master group
then give it a name and then click next over and over until it becomes finish.

once it's created click add again and pick add category
give it a name, then go to the lists tab and add in at least 1 item (I usually just put in "member")

then you can ok out of group setup

then go into view/edit activity groups

pick your master group that you created earlier,

then click add at the bottom,

select your element 1 of member

then press the select people button at the bottom.

then select the people you wish to add to the group, when you click ok it will add them to the group.

You can now search on this group for emails / mailings.