@gc According to https://social.coop/bylaws you become a full member 42 days after the date when you first got your Mastodon account.
There aren't different classes of members on #SocialCoop. Every member regardless of financial status is a part owner of the platform, and can participated in governance, etc.
Of course it is encouraged to contribute if/when/as much as you can, but if you can't that's ok, and the cost of your membership is basically being subsidised by those who do.