"I added you as a friend."
A slightly longer and more normal sentence is “我加了你 为 好友”. 为 (wéi) functions as “as, become” here.
The English slang for this is “I friended you.” but it does not have similar sentence structure. We can only see “我友你” in some awkward/unpopular translation.
There is a non-slang proper verb for this in English which is "to befriend", but it's never used for tech or online stuff for some reason.