Servers can be clustered together in groups of 1, 3, 7, 15, 31 (ie. 2^n-1, where n is a whole number). 

Using the Scimore Enterprise Manager 

1. In the database manager, expand the tree view in the left panel, so that the server group and server name are shown.
2. Right-click on the Cluster node and select Alter Cluster.
3. Select the server you wish to add or remove and click on the or buttons to add or remove servers to the cluster.

Using SQL 

It is possible to add servers to a cluster using the ALTER CLUSTER SQL command. The simplest way to add two servers to make a cluster of three would be something like this:

ALTER CLUSTER (ADD ENDPOINT '10.0.0.2:999', ADD ENDPOINT '10.0.0.3:999')

Similarly, removing servers from such a setup would be:

ALTER CLUSTER (DROP INSTANCE 1, DROP INSTANCE 2)

If the server instances to be dropped had identifiers 1 and 2. This can be checked in the sysinstances system table. 

Related 

• Getting Started
• Installing a New Server
• ALTER CLUSTER
• "sysinstances" System Table