An unorthodox approach to lifting astronauts from the Martian surface is for them to ascend in an unpressurized spacecraft, an idea proposed by the aerospace engineer Frank Eichstadt. The astronauts would need to wear space suits to stay alive, but a significant mass savings could be achieved by using a small spacecraft lacking thick outer walls. The motivation for minimizing the mass of an ascent payload is that it could allow a much smaller and more manageable rocket to lift it into Mars orbit. This is important because the amount of fuel required to transport hardware to Mars tends to be some large multiple of the final mass delivered to the surface. A small reduction in ascent payload mass can have a huge effect on the total mass, and therefore cost, of the mission.
In an unpressurized configuration, crew members could ascend in groups—as is traditionally assumed—or they could ascend one by one. Launching astronauts one by one is another way to allow smaller ascent rockets to be used on Mars. Of course, this would require a separate rocket for each crew member, but the advantage may be simplicity of design.
I think it's safe to assume that before any humans depart for the Red Planet, there will be a successful Mars sample return mission. After that occurs, there will be a number of lessons learned regarding the challenge of Mars ascent. There will be a fully flight-proven design of an ascent spacecraft—a design whose strengths and weaknesses will be scrutinized by Earth's best engineers. If, during a human mission, a single crew member were to ascend in an unpressurized spacecraft, I think it's possible that the ascent vehicle used could have some design commonality with the unmanned ascent spacecraft of the sample return mission.
To provide the crew with breathing oxygen during ascent, they could wear the same portable life support system designed for use with their space suits on the surface. This would be possible if the full duration of ascent—from the time the astronauts don their suits to the moment they enter an orbiting ship's airlock—is less than a surface EVA, or extravehicular activity. Portable life support systems for space missions are typically designed to enable EVAs of at least several hours, which may be just long enough for a crew member to rendezvous with an orbiting spacecraft after an unpressurized ascent from the Red Planet.