To add contacts to an existing audience, the easiest way is to add contact tags.

For example, if you are looking to add a contact to your "Buyers" audience, first check how this audience is governed, e.g: contacts are filtered into this group if they have a "buyer" tag (1).

You could alternatively choose to (add and) target the tag itself, if the tag is the only filter for the audience.

The way to add tags to existing contacts depends on how your contacts are managed in your ActivePipe account:

Add contact tags - Integrated contact accounts

If your ActivePipe contacts are integrated with your CRM, add the relevant tag to their contact record within your CRM. When ActivePipe performs its next sync (allowing approximately 2-3 hours), the contact tag will be updated, and given the tag meets the audience rules, the contact will be added to the audience.

Contacts managed within ActivePipe

If you manage your contacts within ActivePipe using CSV files, you can add an existing contact to an audience manually.

Make note of the tag filter that governs the audience group, e.g: Tag - includes - buyer, as per above.
If the Audience is governed by something other than a tag, you can add a new tag filter to the audience to make this work.

Then navigate to the contacts profile to add the same tag.

Once the contact has the tag the audience is filtered by, they will be added to the audience. 

You can also add new contacts to existing audiences.