Nintendo fan bezzaton on Reddit has created a multiplayer matchup site for Nintendo Switch users in order to connect people who want to play local and multiplayer games. The site is called Switcher.gg and can be signed up via email or Discord.
Once signed up, you can choose to find others nearby for local co-op (within the distance you set) or find others for online multiplayer. The site will then match you to other players based upon games that you both own and other specifications that you can set.
If you sign in via Discord, any messages you receive on Switcher.gg are sent directly to you in Discord, making the process of adding friends and starting sessions easier. An alternative option is to just add your Nintendo ID so people can add you easier, as anyone can view your profile and see what games you play (though only logged in members can send messages).
You can find a link to the website here.