Installing Portainer on existing systme already using docker

Hi, been doing a little searching on the web but can’t find an answer to this one. I am using docker (docker-compose) for a few containers and want to start using Portainer. How does Portainer work when I do this? does it pick up the existing containers and allow me to totally manage them, does it give me partial management or does it not include the existing ones? Is there anything I should know/do if I am installing Portainer on a machine already running containers.