If you would like to join the Libertarian Party, you can join both at the national level: The Libertarian Party, and at the state level: The Libertarian Party of California. You can choose to join either or both. You are not required to be a member of either state or national to register as a Libertarian on your voter registration, nor is it required to join if you want to vote for a Libertarian in an election.
Joining the Libertarian Party of California gives you instant membership to your local county affiliate, such as LP of Alameda County (if you reside in Alameda County).
To be a delegate at the national LNC (Libertarian National Committee) Conventions, one must be a member, in good standing, with the the national Libertarian Party and their state party (i.e. LP of California).
Membership links are here: