First you must make sure that Divisions and Teams are created so that you have Teams to roster to. If you have already done this, skip to step 3 below. 

1. Add Divisions
Go to the Schedules > Leagues & Divisions > Add Division.  Add your Divisions to the League that has the same name as your Registration Program. You can see which League is linked to your Program as indicated by the blue chain link icon. 

2. Add Teams
Go to the Schedules > Leagues & Divisions page.  Use the More Options icon on the extreme right of each Division row to go directly to the "2 Teams" page in a Division.  Click Add Team to add your Team names.

3. Setup Your Assign To Roster Page
After adding the Division and Team names, go to Programs > Manage Programs and click the More Options icon on the right of the row for a Program to go to the "4 Rosters" page.  Click the Assign To Roster button to go to the Assign To page.  Click "Add Filter" to add filters -- and use the Highlight drop down to display data for each assignee (click the Update button after you make Highlight selections to update your page view).

4. Drag and Drop
Drag and drop players and coaches from the left to the right -- you can expand the Division names by clicking the 'triangle' icon to the left of a Division name to see the Team Names.  You can make multiple selections of players by holding down the CTRL key and selecting players (this will work differently if you're using a MAC).

If you want to change out of the drag and drop mode to use checkboxes to make assignments scroll down and click the Toggle Mode button.