You can clone remote repositories from Github, Bitbucket or any git-hosting which support SSH-keys.
In any activeCollab Project, go-to Source tab. You will see Clone Remote Repository option there.
On next screen, you need to enter name for local-repo and remote git-repo URL. If remote repo is public, that is inly thing you will need.
For private repos, please add public SSH-key of PHP’s linux-user to remote repo with read-access. For your convenience, key you are suppose to add on remote-server will be shown in same wizard as you can see below.