ISRCs can be assigned by individual artists, by record companies or other third parties known as ISRC Managers under an agreement — you just need a Registrant Code.
You do not need to be a member of the IFPI or a national music industry trade association in order to apply for a Registrant Code and assign ISRCs to your recordings.
Please remember ISRCs are assigned ONCE and remain the same for the lifetime of the recording.
Should the rights be sold to a sound or music video recording that has an ISRC assigned, the new rights owner MUST NOT change that ISRC.
Source: https://isrc.ifpi.org/en/get-isrc