RIGHT!!
I've just gone through the sequence of magic incantations needed to get ghosts on a league session and got it to work, so I'm going to document it here while I remember...
On the Bkool web site, find the league (Share->Leagues->League name) and then click the stage you want to ride (say Chaingang Feb 2016 - Stage 1 - Baldo Blast) to get the list of results to date. Challenge one of those results or click Copy on the stage page..
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This adds a session to your scheduled sessions list and switches you to the Sessions->Scheduled page in the web site.
Find the session you just added, it should have the league symbol next to it.
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Click on the symbol under the See column for that session and then on the name of the session
This opens the session page, click on SET-UP > in the top right corner and add the ghosts you want
Now before you schedule it click on the Details page
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Under details drop down the list for the type of session and choose Session valid for league
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Then select the league
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Now click schedule in the top right corner...
You should now see a session schedule with the relevant date and the number of ghosts you selected
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Got to the Bsim simulator and find the league tab and the session name and confirm the number of opponents
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Then either start the session or click details to set-up the session (i.e. disable Actual atmospheric conditions) and/or review the Riders
Obviously, make sure that Ghosts are Allowed (and as normal no Solo mode)!
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If you review the riders you should see all the ghosts you chose...
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Start the session and chase your ghosts...
Geoff