Converting a call to a conference/ call between multiple users

We have a mainly mobile workforce using the Ring Central App (Glip app) on mobile phones in the UK.

We have scenarios where an employee will call an external contractor then need to link other internal employees into the call. At this point the call will become a conference call between multiple parties.

Can anyone tell me how this is possible?

Happy to have callers on hold while more employees are dialled in.

TIA.