As a chat group admin, you have a series of additional options. In order to manage the chat group, you just have to go to the information screen where you will have some additional options to the ones regular users have:
1) Add a member: For that, pres the Add members option and select the contact that you want to add.
2) Change the administrator: You can also transfer the group's administration to other user selecting Pass Admin. right next to the name of each of the members and selecting Accept. The selected user will be the new administrator.
3) Expel members: You can expel group members if you want. For that, press the members member you want to expel and select the expel icon.