"Join communism" is found, but it is clear that people had rather use this verb according to its usual meaning ; this is shown in the ngram.
(SOED) join III Come into contact, contiguity, company or union. 14 v.tr. & i. Become a member of (a society, an organization, etc.).

This pattern repeats itself for « fascism/the fascists », except that the new usage is comparatively stronger.

There seems to exist no definition of « join » that is compatible with the concept of ideology, doctrine, idea, etc.
The expression « join a religion » is used, as religion has the meaning "religious order" which connotes that of "organization". However the following are not found.
- join a doctrine, join an ideology, join christianism,join veganism,join naturism,join nudism,join militarism, masonism, isolationism, Maoism, Trotskyism, Leninism,
It appears that there has been in the last century the coming into being of a new meaning for "join" which has not yet been acknowledged by lexicographers, and perhaps will never be.
The idea of more or less gradually becoming a supporter of an idea, doctrine, ideology, political line of thinking, or religion is rendered faithfully and generally enough by the following verbal expression.
became a convert to communism (examples)