"Communication" is the process of exchanging ideas, opinions and information. There can be different processes (plural) but each process is a set of regular steps or actions taken to achieve a particular end and is considered singular.
There are several processes (plural noun) such as germination, pollination and fertilisation in the life cycle of a flowering plant. However, "germination" is singular (there is no such word as "germinations"). // The reproduction of penguins NOT The reproductions of penguins.
Back to your sentence, it should be written as: "With reference to the communication/correspondence you have had with my colleague John, please drop by on Friday at 10.30 a.m. for a discussion."
I hope this clears the air.