How do I send emails to only certain people within a campaign?

You can either "select" certain donors (first example below) or "filter" for a set of donors (second example below):

1.
Click on Emails link from left-hand navigation panel of OPPS.
Then "Add a new email".
Provide a Description for this email, as well as From Address, Subject, and Body.
Click the "+" in front of Recipients (default says "all donors")
The field will now say "Select Donors", with a secondary dropdown that offers "Include all donors", "Include only selected donors", or "Include all donors except those selected".
Choose one of those options, such as "Include only selected donors".
Then double-click on the name of the donors to move them from the left field to the right field.

2.
Click on Emails link from left-hand navigation panel of OPPS.
Then "Add a new email".
Provide a Description for this email, as well as From Address, Subject, and Body.
Click the "+" in front of Recipients (default says "all donors")
The field will now say "Select Donors", so click the dropdown and change to "Filter Donors".
The screen will refresh and show you options such as "By Donor Group".
Choose one of those options, such as "By Donor Group".
Then double-click on the names [of the donor group(s)] to move them from the left field to the right field.
You may use more than one filter type to further restrict your choice of filtered donors.

Lastly, click on "Preview" at the bottom of the page to see a popup with an estimated count of donors.  Keep in mind that this will plan on sending only to people who have email addresses populated on their donor record.  You can then Save/Queue, or Save/Update to work on email later.

Example 1:



Example 2: