Login to your Exotel account and click on "Co-workers & groups" tab on the left and "Invite Co-workers" button on the top right as shown below:

 Note that the default number of co-workers (maximum) that could be added is 50 and in case you wish to be able to add more, please reach out to our support team at 8088919888 or write to us at hello@exotel.in and we shall assist you henceforth.

Video reference attached.