I just wanted to check before using this, we already have bases for EasyBase1 - 5 etc. But we want to add in more bases. When using rb.config add do we have to readd the existing bases  e.g.

rb.config add "easy bases" easybase1 easybase2 easybase3 easybase4 easybase5 easybase6 0

or does this just add the new one e.g.

rb.config add "easy bases" EasyBase6 0

I am terrified of messing up my config as its all working nicely at the moment. Any help appreciated!

You can very easily add your base to the file manually if you wish.
the file is /oxide/data/RaidableBases/Profiles/EasyBases.json
It looks like this for each base...
"easy_4": [
{
"Option": "stability",
"Value": "false"
},
{
"Option": "autoheight",
"Value": "false"
},
{
"Option": "height",
"Value": "1.0"
}
]

Note the last base must not have a comma after the ] all the others must

I'd prefer to be able to use the command if possible as its another admin making bases and he doesn't have file access. 

you can use it to add bases to an existing profile without having to re-add all of the existing bases again. it can also be used to create a new profile if it does not exist