You add them by searching for their account address (e.g. https://pixelfed.social/earth) and then clicking on follow.
However, currently this doesn't work with Pixelfed instances because of a bug. You can follow Mastodon accounts from Pixelfed though.
If the account isn't visible, you can search for the account's URL in the Mastodon search box to bring up its follow button.
This also works for other fediverse accounts such as PeerTube, Pleroma, Write Freely etc.
For example, try searching for https://pixelfed.social/earth
Bobinas P4G is a social network. It runs on GNU social, version 2.0.1-beta0, available under the GNU Affero General Public License.
All Bobinas P4G content and data are available under the Creative Commons Attribution 3.0 license.