I say "Come see us sometime." and "Come see us sometimes." are both good with a different meaning. 
The second meaning "once in a while". A friend tells me that you can't write sometime(s) with a 's' in that sentence.

Who's right?
Hi claudej, and welcome to English Forums.

Your friend is right.

Sometimes, they come to see us. --- That would mean "Once in a while they come" (habitually)

Come to see us sometime. --- At some point in the future, on a non-specific date, come see us.
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You mean I can't say "Come see us sometimes." even if I want to convey the meaning of "once in a while".

So I'd better use "Come see us once in a while.".

Another rule without a reason ...
Yes, or "from time to time." Or even "Don't be a stranger!"