Yes, you have to enable it (that will modify the samba configuration file).
Right now your XP and Win2K clients can access but are prompted for passwords. Unless you set up the same user on SME as the user you have in your Win98, XP and 2K,... you'll get prompt for user/pass, otherwise not.
There are many advantages of logging to a domain, such as security and "one password for all".
Your windows XP and 2K will add the domain's profile and you'll have a choice of logging in locally, or to the domain.
On a side note, XP needs to be patched with a file located at
http://YOUR_SME/server-resources/so it can join the domain.
Hope this helps,