You can easily add people to a group who are already on an Email List, or already filtered in a saved report that you run regularly.
First, create the group you will add the people to if that group does not already exist. You must create your group first, as you will need it later for this process.
To add people to a group in an existing Email List, go to Reports > New > Add People to Group (under the People section).
In the Options column on the left, choose a join date (this will become the group join date for the people you are adding - can be today's date), the group that the people are joining (the one you just created), and the group role (typically this is Member).
In the Filters column in the middle of the page, click the blue People filter bar, then People Attributes. Scroll down to "Email Lists" and choose "Select" and the Email List in the box just below (see image below).
Click "Go!" in the Review column on the right. You will have an opportunity to first display all the names that could potentially be added to the group, before actually adding them. If you are satisfied with the results, go forward and click "Add to Group."
To add people to a group from a Saved Report, go the Saved Report, where the filters are already set. (Reports > Saved, then select the Report you are wanting to use).
Click the "Change" button on the top right hand side under the Review column (see image).
Now, you are able to choose a new Report, but the filters of your Saved Report are kept.
On the left under the "People" Section, choose the "Add People to Group" Report.
In the Options column on the left, choose a join date (this can be today's date), the group that the people are joining (the one you just created), and the group role (typically this is Member). All of the Filters should be there for you in the middle column, so you should not need to make any changes.
Click "Go!" in the right column of the page. You will be given an opportunity to preview the names of those that will potentially be added to the group. If you are satisfied with the results, proceed with adding to the group.