When looking at the list of members in https://github.com/orgs/plone/people it looks like only about half of the members are publicly visible. There is nothing wrong with that: you can keep your membership private. But I am guessing that this is simply the default setting and not a conscious decision by new members.
Why would you want to make it public?
- Plone is then visible in the list of organisations on your own public profile page.
- People who review pull requests can go to your profile page to quickly check that you are a member. If you are a member, this means you have signed the contributor agreement, so that is one hurdle less for merging the pull request. Otherwise we have to go to https://github.com/orgs/plone/people, and filter on your name to see if you are there anyway.
Number 1 is the most important reason.
How do you make your membership public? Go to https://github.com/orgs/plone/people, filter on your github name, and change the dropdown from private to public.
Again, it is perfectly fine to keep your membership private. But if you are happy making it public, then go ahead.