Neither are very good for the intended purpose. The following is better:
We'd love to have you work with us as a helper.
Anyway here are some brief comments on your original sentences:
We'd love to see you join us as a helper.
This is correct English, except that it is strange to say that we would love to see people join if we are part of the organization that we would like them to join. It makes it sound as if we are merely observing them from a distance rather than welcoming them.
We'd love to see you joining us as a helper.
This is even stranger, because "joining us ..." now refers to the action of the joining itself. I would say that the sentence isn't quite correct in English because the semantic meanings of "join" and "as a helper" are incompatible. To see more clearly why this is so, consider "he joined us as a helper" and "*he was joining us as a helper". The former has "as a helper" specifying what "he joined us" as, but the latter is clearly incorrect because you cannot use "as a helper" to specify how "he joined us".