All speakers were confident, comfortable with being seen.

with 是介词，being seen 是动名词（被动形式）作介宾。

be comfortable with...的含义：comfortale 是“舒适的”，with 指“原因”。字面：因......而舒适。因某事而舒适，自然会满足于某事，乐于做某事。引申为“乐于被人看到”。

All speakers were confident, comfortable with being seen. 所有的演讲者都很自信，乐于被人看到。