As you create a social media post in Apostle and reach Step 2: Choose Channels, you'll notice a star button next to the names of your ambassadors:
By selecting the star next to an ambassador's name, that individual will receive the post suggestion as a priority, receiving it first. The remaining ambassadors will receive the post later within the time period you've chosen.
Please note: The prio star will only work if you use a Date Range whilst scheduling your post in the final step.