Questions Is there any impact to existing license if we add new company?

Assuming you are using version 2018 R1 or later, there is no impact on your license by creating new companies or branches. You are permitted to have an unlimited (theoretical) number of companies and/or branches.

If you are using a version prior to 2018 R1, then you are limited to the number of companies (tenants in version 2018 R1 and later) you are allowed to have. (from 1 to 10 by default. More can be purchased if needed)

Thanks for your reply. We are using version 5.3.14 and our licence still can hold 4 more companies (we have 6 companies existing in the license already) .  I would like to know if i add a new company, will there be any impact to existing license? Also will the new company inherit the license automatically? 

Licenses are shared by all Tenants (what you are calling Companies). 

Thank you so much for your reply. 

So which means, if i add a new tenant (company), the tenant will auto-inherit the license correct?  Is it, I no need to key in the license number for the new company?

By adding a new company, is there any chance, the existing licence can go off?  

Licenses are at site level. You can add new Tenants and don't need to worry about licenses, other than allocating them to the users in the Tenant. Users are Tenant level, but the licenses you allocate them are still Site level. 

I can't remember what version this was introduced in, but this ACTIONS -> Change to Test Tenant option should allow you to turn what is now called Tenant (but called Company in your version) into Test Tenant/Company that doesn't count against your license and can only allow 2 Concurrent Users at a time.

Unless of course if you are a MYOB Advanced customer in Australia or NZ in which case it still counts against your 5 Tenant limit and so is therefore pointless.

That's a bummer 🙁