Learn Finnish: Muutama kannattaja liittyi järjestöön. - A few supporters joined the organisation.

In Finnish, quantifiers like muutama (a few), moni (many), and usea (several) are often treated as singular words. When they act as the subject, both the quantifier and the noun it modifies stay in the nominative singular. So, it is literally written as a few supporter.

Since it means a few supporters, why is the verb liittyi singular instead of plural?

Finnish verbs must agree grammatically with their subject. Because the phrase muutama kannattaja is in the singular form, the verb must also take the third-person singular form (liittyi), rather than the plural (liittyivät).

I know that numbers usually require the partitive case. Does muutama behave like a number here?

No, and this is an important difference! While numbers force the following noun into the partitive case (like kaksi kannattajaa), quantifiers like muutama, moni, and usea agree with the noun in case and number. That is why we use the nominative muutama kannattaja instead of a partitive form.

Why does järjestö have the illative ending -ön here?

The verb liittyä (to join) requires the noun to be in the illative case (the into case). In Finnish logic, you join into an organization, so järjestö takes the ending to become järjestöön.