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 brand ambassadors:

By selecting the star next to an brand ambassador's name, that individual will receive the post suggestion as a priority, receiving it first. You can select the priority star for multiple brand ambassadors. They will then all receive the posts first. The remaining brand ambassadors will receive the post later within the time period you've chosen.