If you know the exact size of the league system you want to setup and want to invite league members into specific leagues that you create, please follow this tutorial.
Step 1: Create your company
You must be logged in to your Sleeper account to create a company. Once you are logged in, go here:
Congrats, you are now in your dashboard!
Step 2: Create a league group
Before you can have a league, you must create a league group. All leagues in the league group when created will copy scoring settings and rules from the league group.
Click on "+ Add a managed league group" at the bottom of your dashboard.
Now fill out some basic settings (you can customize this later).
Now your dashboard should look like this:
Step 3: Create Leagues Manually
Above your league group, you'll see a view Leagues in this group link. Click on that to open up your leagues.
From here, simply click on "Create League".
You will now see your league here:
Click your league to view it.
You'll see a private invitation link that you can send out to specific people to join that league.
During the season, you'll be able to see matchups, standings, and more by viewing any league here.