Once you’ve created a group, you can always add members to it or remove them from it. There is no cap on the number of participants in a group.
First, select the group you would like to manage from the Chats tab in the Navigation Panel. Then, click the group’s name at the top of the main panel.
Finally, click add members at the top of the screen.
Select any contacts you would like to add to the group and click Confirm to save your changes.