Nintendo of Europe opened a site for Splatoon 2 community tournaments late last year, with the first Community Cup to be held on January 13, 2019.
The site itself hosts news on tournaments, and you can also register for Nintendo-hosted community cups there. For people looking for a team, the site also offers an option to send in details so you can be paired up. You can find the site via the link here.
The first Community Cup is to be held on January 13, 2019, with 128 slots total available. Registrations are currently open and ends in four days. The site also offers an invite to the Splatoon 2 Europe Community Cup Discord, which you can find via the link here.
Splatoon 2 is available on Nintendo Switch.