@alxsim There might be some useful examples (or not) in my list https://codeberg.org/fediverse/fediparty/wiki/Collectively-owned-instances .
Looking at biology journals in DOAJ, I don't see many open-membership associations... Perhaps
* https://doaj.org/toc/2469-0228
* https://doaj.org/toc/2427-6111
* https://doaj.org/toc/2179-975X
* https://doaj.org/toc/1667-782X
of which only https://www.asaeargentina.com.ar/como-asociarse seems to have a procedure for membership from outside their host country.