Does Soul Memory Affect Co-op or Only Invasions?

Solution 1:

Namco has a knowledgebase article stating it does.

http://support.namcobandaigames.com/index.php?/Knowledgebase/Article/View/420/0/dark-souls-ii---im-having-trouble-with-summoningsinvasions

Total amount of souls collected and levels - A character's total number of souls is factored in during summonings/invasions (generally you must be within 50,000 souls of the person you're invading/co-opping with and your level must be within +/- 10 levels).

The 50,000 souls requirement seems to be completely wrong, though.

According to fextralife:

http://darksouls2.wiki.fextralife.com/Summon+Range+Calculator

1) Your soul memory plus or minus 25%. 2) Your Soul Level plus or minus 10, AND THEN a further plus or minus 10%. Both 1) and 2) must comply between the two players for online play. When using a name-engraved ring the percentage of souls allowed jumps from 25% to 30%

Solution 2:

I have had some problems with soul memory. I gamed with a friend perfectly throughout the day then all of a sudden we could not summon each other. The difference was I was using a ring which gave me more souls. (it might be soul memory or a coincidence, will do some more research)
We had the exact same soul level, using God rings, regional lock was off (we live in different regions) and communicated by pc to ensure we were putting the signs where we needed. So yes I believe soul memory does effect Co-op. (again it might of been an annoying coincidence that we could not Co-op once my soul memory became larger)