I was wondering if its possible to have multiple companies on 1 Asterisk server and if so is there any guidance on how to set this up?
Yes it is possible, but you need to provide more details on the scope of what you are trying to do. As the recommendations could go in many different directions.
Well I was thinking of having one server which is situated at our offices and then connect multiple companies (at various locations) to the asterisk server (our offices) and then setting each company up to use the PBX VOIP services provided. If that makes sense???
Everything is seperated by contexts. So that pretty much takes care of running multiple companies on one system.
Well, except for call parking. 
Remembering that there are limitations to the number of users on a particular server.
If you’re only thinking of 1 server, the recommended max users is 15 even for a 3Ghz machine with 1G of ram.
Multiple companies? Are they really, really small?