'Where are you up to?' and 'I'm up to...' can also refer to how much progress you're making with a particular task. For example, if you are reading a long book, you'd say;: 'I've been reading this book for months, and I'm still only up to chapter 3.' So, one situation where you could say 'I'm up to ..(someone)..' is if you are working through a list of people. If someone is in an office sending out letters to people on a list, they might say 'I'm up to John Smith', meaning that this is the stage in the job which they have reached. It would have helped us all a lot if you had given us some more context, Yubo!

Another meaning of "be up to" is to be "good enough" or "up to a standard". If I say "I'm not up to that job", you don't have the ability to do the job. Also, to "go up to someone" means to approach them. But you can't say "be up to someone" in either of these meanings either.

"Up to" is a slang term to refer to what you are doing. Example: What are you up to? Meaning: What are you doing now? You can also use it to describe behavior. Example: He is up to no good. OR He is up to mischief. Meaning: He is doing something bad. But I've never heard this phrase used, "be up to someone." It's possible that what you read was a mistake, or that the person used the term incorrectly.