I use a GB ROM on my Infuse 4g but it does not have CIFS built in. I am told there is an external way of adding this in. Does anyone have a link to this?
What do you want it for?
There's 3 ways to get CIFS client support on Android:
With 2 & 3, you still need some way of loading/activating the module and mounting shares. This might involve editing boot scripts or loading a 3rd party program (e.g. Mount Manager) to handle the mounting/unmounting. Even then, not all your applications will nicely handle the paths to files on a network share.
Now, if you want to serve CIFS shares out from your phone, that's a whole different ball of wax, and there's a number of apps in the Market that will handle that.