Once the job is created, click on the Outsourcing tab and then on 'Outsource the job'.
In the popup that opens, adapt the quantity of the job to reflect what the first supplier will handle (how many words they will translate for example), and click on 'Search for suppliers'.
The system displays a list of suppliers providing the service concerned. Click on 'Outsource' next to the person you would like to choose.
Then, in the Outsourcing tab of the job (where you can now see your first outsourcing for this job), click again on 'Outsource the job'. Again, change the quantity to reflect what this second supplier will handle, search the supplier in the list and click on Outsource.
And so on with all the other suppliers that will work on this job.
About the Quantity field, please note that the system doesn't automatically calculate the remaining quantity based on the quantity you defined for the other suppliers for this job. You have to indicate yourself how many words (or lines or hours, etc.) each supplier will handle, as the system will always mention the whole quantity again.
You can add as many suppliers as you want for each job.
In the list of jobs, you can see little icons on the right indicating that the job has been outsourced to suppliers. You can see the details when you hover the mouse over these icons.
To discover how to use LSP.expert's supplier portal, read this article: http://help.lsp.expert/en/articles/3340222-getting-started-with-the-supplier-portal