To allow an IP for the system you have to log in with the managers account and go to the Settings page by clocking on this button:

Then scroll down to the Firewall section and click on the green plus. Add a "Description"(For example the name person that will use the IP), enter the IP in the "IP Address" field.
Then select the "Port Type". You can allow the IP to have access only to the web page, only to use a soft/hard phone or for both.
Then click on the green plus on the right side of the row:

And you are ready :)