You can only invite new members to a group when you have got enough rights. Whether you have those rights depends on the settings of the group. If the owner of the chat (the one who started the conversation) has changed the settings so that only he / she can invite new people you will have to ask the owner to do it for you.

If you do have the required rights, in the web interface you will see a green circle with a white plus in it above the list of group chat participants on the right. When you click on the plus you can pick contacts and / or colleagues to invite.

On your mobile device you can invite new people by going to the menu in the top right corner of your group chat and 

choosing ‘Chat options. You can then click ‘Add members’ in the ‘Members’ tab.