A group admin or a group owner can add participants to an existing group. If you are not a group admin or group owner you will not see this option.
To add a new participant to an existing group:
Locate the group to which you wish to add new participants.
Click on the group name or profile picture at the top of the chat window.
Or
Click on the ellipsis icon and then click Group info.
In the Group info screen, click Add participant.
In the Add participant screen, select the participants you wish to add to the group and then click Add participants.
The participant is added and displayed in the group.