Official AT&T guide: (hot trash)

https://asecare.att.com/tutorials/officehand-conference-calling-frequently-asked-questions-3993/?product=AT&T%20Office@Hand/ 

Log into the Office@Hand app or website (see below) with your Gmail credentials.  Click the icon circled in red (step 1).  The mobile app has the same Icon and is located in the top right corner.

Click the conference option (step 2) to have the information displayed.

This is an example.  Make sure to check that the number will be right for you.

Click the “Invite with Email” to invite with Email.