Does the entire crew fit into the Soyuz life boat they have today?
Each crew member comes with a Soyuz which remains docked and can be used as a shelter or to return to Earth (actually the oldest docked Soyuz returns first, the hydrogen peroxide of the reaction control system degrades with time).
- For example: TMA-11M (carrying the Olympic flame...) docked to the ISS on Nov. 7, 2013 with 3 additional crew members, making the total aboard to 9 persons. At this time 3 Soyuz were docked: TMA-09M (initially on Rassvet but relocated on Zvesda to make room for TMA-11M), TMA-10M (on Poisk) and TMA-11M (on Rassvet).
The relocation of TMA-09M was done with its 3 crew members onboard.
The same crew members (and the flame) returned to Earth with TMA-09M on Nov 11.
Do they have to get dressed and prepare in other ways?
Most of the equipment of the ISS can be operated remotely from the ground, so there is not much to prepare before evacuating. The astronauts and cosmonauts put their suits on while in the Soyuz.
The Soyuz vehicle can be used even if the communications with Earth are lost. The crew of three inside it can manually return to Earth by themselves.
A Soyuz can be separated from the ISS within three minutes. Typical time from decision to leave until landing in Kazakhstan is just under 3.5 hours (source).