In order for you to add Users/contacts to existing WhatsApp groups, you need to be the Group Admin. If you are not the Group Admin, get in touch with them and forward them the contact that you would like to add.
To add a contact to an existing WhatsApp group simply do the following:
1. Open WhatsApp
2. Click on the WhatsApp group where you would like to add the contact
3. Click on the title at the top
4. Scroll down to the bottom of the participants in that Group
5. Click on "Add Participant"
6. Select the contact that you would like to add and you're done.
Remember it is common courteous to ask, or advise all group members that you are adding someone new to the group, as well as seeking consent from the new contact that you are adding them to a new group.