Google Photos is rolling out the ability for you to tag someone in a photograph manually. The app already can recognize and sort people in photos, but now, if it misses someone, you can fix it yourself. The catch is it won't just let you tag any person in the photo willy-nilly. Google Photos needs to detect that there is a face there first. If it spots someone, then you're good to go. But sometimes it won't recognize faces that are slightly turned or blurry. Android Police has a comprehensive guide on how to use this new feature.
If the feature hasn't made its way to you yet, you might need to wait for a bit. The update seems to be part of a slow rollout.