If you would like to join the Libertarian Party, you can do so 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 Party Member 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 The Libertarian Party of California gives you instant membership to the State party plus your local county affiliate. If you reside in Alameda County, the Libertarian Party of Alameda County would be your affiliate.
To be a delegate at the Libertarian National Committee Conventions, one must be a member, in good standing, with their State party (Libertarian Party of California) AND National party (The Libertarian Party). Note: National membership does not include State or County membership).
Membership links are here:
Libertarian Party of California: https://ca.lp.org/membership/
National Libertarian Party: https://my.lp.org/contribute/join/