Hei 🙂
As Sebastian stated, for a domain join to function the Host will have to have the client running.
Information here https://wiki.fogproject.org/wiki/index.php?title=FOG_Client
And for further questions, have a look around the wiki first, it will answer many questions. Anything not listed there people on the forum will be able to help you with. The Developers are VERY active and great 🙂