Generally you should add new players on the Sport Ngin site. However it is possible to add them on the iPad app.
Go to the game you need to add a player in (it will be available for other games using the same team after added).
Select the roster button for the team that needs the player added.
In the lower right corner select the player button with the plus '+' indicator.
You will get a notification about the web site. If you have pushed stats you need to do an "Admin Game Reset" on the Sport Ngin site for this game in order to get the stats posted for the new player. This is a limitation due to adding from the device instead of adding from the web side and doing a 're-sync' of the roster instead.
Selecting "Allow Add" will present the screen for adding the new player.
Note there is no player ID because the web side will assign that (you do not need to know that detail but pointing out to answer those who know how this information is used).
Save the changes to return to the roster.
You are now ready to use the new player and will need to do the aforementioned admin game reset prior to posting stats to see the new players information.