There is at least one chat administrator per chat who can edit the chat including participants, manage the role assignment within the chat or delete the whole chat.
The number of participants in the group chats is not limited. If you are an administrator of a group, you can add as many people as you want.
If you are not an administrator, you can contact another administrator and ask him/her to add a new person to a group. You can find out who is the administrator of a group by clicking on the image of the group chat and then on "Participants". Close to the name of the administrators you will see the addition "-Admin".