Conferencing 4+ without a bridge

How can I conference in more than 2 people without using a bridge?

On my Mitel system I can hit conference each time I want to add a person to call the call…

My Yealink T38G limits me to only two.

You can’t, your Mitel has a bunch of “Confrence resources”, the Yaelink has two, as do most ip phones, asterisk uses meetme or confbbridge to provide the audio mixing/balancing.