Clusters of electrodes are mostly there for display purpose.
They can be useful for overlaying one electrode over multiple files:
https://neuroimage.usc.edu/brainstorm/Tutorials/ChannelClusters
To get the cluster's time series, you have two options:
- create a cluster, use the process Extract > Clusters time series, then compute what you need on the resulting file
- Use a montage and the process "Standardize > Apply montage" to create a new signal in the files. Do not try to do any source modeling on the files you have modified in this way.