From the +Post button, select your audience by tapping the Audience drop-down on the left-hand side. Your options will be determined your permissions.

Click the Select Audience to filter your Audience.

Note - Your default Audience will be based on your last Post Audience.
Example: If you chose to send a post to the Chicago Location in a previous post, when you create a new post it will automatically default to Chicago for your next Post Audience.

-Suggested: My Location, Team, My Department, Everyone (if your company allows)
-Recent:  Most recent Post Audience
-Personal Lists: ( If created by user)
-Groups: List of each Group you have joined.
-Custom: Select colleagues individually through a list or use Custom Filters to select particular sets of colleagues.
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Custom Audience:

Choose Specific People

Or choose from any of the Advanced Filters setup for your Company.