Here are a few other options to consider:
1. If you want to stay at the Charles, you may be able to find someone who already has a reservation and is looking for a roommate. Roommate matching is taking place on CrowdVine.
2. This google map shows the Charles Hotel and the symposium venue (Wasserstein Casperson Clinical aka WCC). As you look for options, consider adding the addresses of potential hotels to this map by clicking "add destination". This will allow you to see where a given hotel is in relation to the main symposium sites.
3. Hotels.com had several good options (including the Marriott Courtyard for $169) at the time of this writing. Enter the city as Cambridge, MA then be sure to then narrow selections by neighborhood (Cambridge) otherwise many out of area options will be shown.
4. A number of people are staying at the Irving House. This bed and breakfast appears to be booked but a note on their site says to call as they may have something available.
See you soon!