Tap on the magnifying glass at the top right of your screen (Figure 1):
Figure 1 Search
Narrow your search results by People, Rooms, Messages, or Posts. Matching information will be displayed as you type.
Find and connect with external contacts
Use Search to find and connect with people outside your organization. Tap on their name to view their information. Tap the message button in the bottom right to send them a Connection Request (Figure 2):
Figure 2 Connection Request
After they accept your request, you can start chatting.