Ottspott allows to add participants (as many as you wish) to an audio-conference call. For each participant you wish to add, just click on the green button on the bottom of the call page.
You can invite participants in 2 ways:
Call URL: just click on "COPY" and paste the link in another Slack channel, direct message or e-mail.
Call to a cell phone or landline: just enter the number you wish to call.
Example for a number in San Francisco/USA, type 16507410003 or +16507410003 .
Each time you add a participant, you can choose either the URL or the number method.