There are certain rules for combined teams.
In Sweden, i.e. Tiomila, you can legally combine runners from three clubs to make a team. The combination has to be announced in advance and you can´t just pick any runner on the day.
Last year there was a well publicized international womens' team with elite runners from more clubs (and countries) than three. There was actually some discussion with the organizers before they were allowed to run.
At Jukola they have different rules - I don´t know for sure about combined club teams, but the vast majority of teams ~1000+ are so called corporations made up of families, company teams etc. Those teams are basically not allowed to consist of "active competitive orienteers".
Long ago (in 1981) there was actually a Swiss
national team competing in Jukola. They were not included in the final result list but would have ended up in top-10.