FIFA 18 on Switch currently has no way of letting you play with friends online
FIFA 18 does offer online play on Switch. Unfortunately though, the game has a huge omission. As the game is now, you have absolutely no way of playing with friends online. The only way you can run into a friend is through random chance.
EA patching FIFA 18 to support the feature may not be entirely off the table. But as the game stands now, that is a significant feature to be lacking.
Other elements missing from the Switch version of FIFA 18 compared to its console counterparts can be found here.