If they are in the same network, they can access your Redmine installation (you need to enable the firewall in your Mac OS X). If they are not, you will need to configure your router to forward all the incoming requests from your public IP to the private IP of your computer.
The computer must be running and the different services of the Redmine stack too.
As I mentioned before, you will need to configure your router to forward the requests in case the users are outside your network. This is a generic information about how this works (you will need to find the specific documentation of your router)
You do not need to allow remote login, you only need to configure the firewall under the Security & privacy settings
No, this is not needed
I hope this information helps