You can add participants to a channel in 3 ways:
Participants count icon
Actions
Textbox command (+)
1. Participants count icon:
In the channel header, you will find an icon representing the number of participants in the channel. To add more, click the icon. 
A dialogue window with an add participant button and a list of channel participants will appear. Select Add Participants. In the dialogue window that appears, select the participants you want to add and click the Add button.

2. Actions:
In the chat window, select Actions and Click Add Participants. You can select participants from the list of suggestions.
3. Textbox command (+):
The quickest way to add participants is by typing '+' in the textbox. A list of participants will appear on suggestions. Click on the participants or move your arrow keys and select Enter.
Note : Admins can add participants, but other participants can only do so if they have permission.