Rennes is located about 350km West of Paris. The easiest way to come is to first go to Paris, then take domestic transportation to Rennes.
The most convenient way to reach Rennes is by train. French trains are fast, safe and comfortable. Any purchased train ticket includes a seat reservation (slightly more comfortable in 1st class, slightly cheaper in 2nd class).
Note that train tickets must be purchased as early as possible, otherwise you’ll end up paying much more than necessary and risk not being able to board. Train ticket reservations open roughly three months before the chosen travel dates. Local metro between the Paris Orly airport and the Paris city center can be purchased at the last minute.
Cost-conscious attendees may want to take a look at long-distance bus lines to Rennes.
There is a small airport in Rennes (airport code: RNS) with a few direct flights from cities in France, UK, Belgium, and the Netherlands. There are also 2-3 direct flights from/to Paris CDG airport.
Driving from Paris to Rennes takes about 3h, with a one-way highway toll around 32€.