Hi Daniel,
As far as I remember, the form said "Join the Fellowship" and explained that this was a financial contribution.
If you go to https://fsfe.org and click "Become a supporter" it
still shows the same form with the title "Join the FSFE"
The word contribution is very generic. In my view, the word itself
doesn't strongly imply membership, but the act of giving money does
give many people the feeling that they are a member.