If a rider is ejected for doping, can a person from that same team still finish on the podium? I'm thinking specifically about TTT & any advantages a "boosted" rider would provide to a GC.
Riders aren't ejected for doping they leave 'voluntarily'. The B sample results are never in by the end of the race so technically they still haven't tested positive.
The answer to your question is yes team mates of a rider whose A sample tests postive could end up winning the TdF. If the B sample was also positive they would still be the winner as far as I know.