The process of adding a node to an existing Cassandra cluster ranges from trivial when vnodes are used to somewhat tedious if you are manually assigning tokens. Let's start with the manual case, as the vnodes process is a subset of this.
As previously mentioned, the procedure for adding a node to a cluster without vnodes enabled is straightforward, if not a bit tedious. In general, you should add one node at a time, unless you're able to double the size of the cluster. Doubling removes the need to reassign tokens, as Cassandra's default of bisecting another node's range will be sufficient. The first step is to determine the new total cluster size, then compute tokens for all nodes.
To compute tokens, follow the DataStax documentation at http://www.datastax.com/documentation/cassandra/1.2/cassandra/configuration/configGenTokens_c.html . There are also several useful online tools to help you, such as this one at http://www.geroba.com/cassandra...
