How to add new column family to an existing HBase table?

alter 'tablename', NAME => 'newcolumnfamily', VERSIONS => 50

you can specify various properties of the new column family separated by a comma (,)

It seems

alter 'tablename', 'columnfamily2'

does the trick. One may, disable 'tablename' first. However, it works fine even if enabled.

hbase(main):015:0> alter 'tablename', {NAME=> 'columnfamily2'}
Updating all regions with the new schema...
0/1 regions updated.
1/1 regions updated.
Done.

If you consider some GUI solutions than you can easily do it with HAdmin. Simply press the "+" button in the "Alter table" page and add your new column family with all settings you need.