I'm not convinced that "Support us" is the right name for this page but it'll do for the moment.
I would open with a paragraph saying volunteering a free means of support but there are running costs to be covered, projects need funded, etc and we are a charity so if you're unable to volunteer then you can still support us by becoming a member/affiliate. Rubbish wording but that's the general gist of it.
Membership/affiliation will be in focus on the home page but will ultimately link here (or to a sub-page of here).
I would include an option to "make a donation". Having it here means it exists but it's not right in your face desperately begging for donations as it would be on the home page. Instead, it's existence can be referred to/hinted at in comments on the home page, newsletter, etc.