Not everyone with Switch has Nintendo Online, but Pokémon Sword and Shield has many fun ways to play local multiplayer with friends around you.
The local Y-Comm game can be conducted no matter where you are in the Galar region, while the rest takes place specifically in the wilderness area.
Link Trade (Y-Comm)
Link trading is when you and a friend choose a Pokémon to trade with each other. This is especially useful if a person plays Sword and the other Shield games because you can trade version-exclusive Pokémon and run out of your Pokédex. Some main exclusives are Cursola and Galarian Ponyta (Shield) and Sirfetch'd and Stonjourner (Spada).
Link Battle (Y-Comm)
Link Battling is when you and a friend put together a team of Pokémon and fight against each other. You can choose between Single, Double, and Multi-style battles, and also choose whether to play with your Pokémon at the levels they are in, or flatten them all to level 50 if you and your friend aren't close enough in the level.
Exploring (wilderness area)
If you and some friends are playing Pokémon Sword and Shield at the same time, and you're all going to the same part of the Wild Area, you'll find you can see each other. There's usually a bit of a delay, and sadly, it won't show up on any of your friends' screens if Charizard poses, but other than that, you can freely explore the wilderness together, finding berries and Pokémon.
Camping (wilderness area)
If you place your Pokémon camp in the wilderness, your friends can see it. If they stand on it and press A, they visit your field. You can talk / play with your Pokémon and those of your friends and you can make curry together. While only one player can choose the basic curry ingredient, everyone can add berries, making cooking less of a strain on your individual resources.
Max Raid Battle (Wild Area + Y-Comm)
Around the wilderness there are Pokémon lairs. Inactive lairs give you 50 watts, while active lairs give you 300 watts and the chance to fight the Pokémon Dynamax inside in a raid-style battle. You can do this with NPCs or up to three friends to form a team of four. If you're playing with a friend or two, the rest of the raid trainers will be NPCs. If you win, everyone has a chance to catch the raid Pokémon and a variety of prizes.
Note: Raids will take place in different lair locations and at different levels for each player. When you find a raid, you must invite friends to fight with you via the raid menu. They can join you by opening the Y-Comm, updating the feed, switching to your invitation and clicking "Join".