I originally joined by being randomly posted a questionnaire with an explanation about what they were doing and that I'd been selected from the electoral roll (and a blurb explaining they are allowed to do that 😅). These days it's done online, that was 10+ years ago that I joined. They literally just have the link to the questionnaire on their website, but they are doing a 20 year study of changes over time so I'm not sure if they have any new people joining at this stage - I have no idea what would happen if you just filled in the questionnaire!

On their FAQ they have this slightly relevant question (emphasis mine):

Can my partner join the study?

The NZAVS was designed to randomly sample adults from all across New Zealand. Randomly sampling people means we can provide reliable estimates of the overall population. This remains the key focus of the NZAVS. Over the years, a number of participants have also let us know that their partner is interested in joining the NZAVS, and to ask if their partner could also be sent a copy of the questionnaire. Partners who join the NZAVS are not part of the original random sample, and because they are choosing proactively to join the study, they might differ in important ways from the more general random sample of participants. However, including partners in the NZAVS allows us to answer additional and important questions about how people affect one another in relationships. For example, people’s own attitudes and values might predict their own wellbeing, and their attitudes and values might affect their partner’s wellbeing. To examine how partners’ influence each other like this, researchers need data from both people in a relationship, not just individuals. This new exciting part of the NZAVS will use the data we have from both people in relationships to understand the ways that people’s attitudes, values and wellbeing develop within, and are affected by, their closest relationship partners. If your partner wants to join the NZAVS then they are most welcome to do so, and can join the study simply by completing the questionnaire online and entering their contact details.

It sort of implies that they need people to join by random sampling to make sure they get the right spread of people, so I guess you can't choose to join? You could always email and ask, they have contact details here: https://www.psych.auckland.ac.nz/en/about/new-zealand-attitudes-and-values-study.html**___**