Some users have reported that they have been added as a friend and told that their account are pending to be "banned". The account in question could be someone they never encountered before, or someone on their friend’s list for a long time (presumably hijacked).
They will request that they are to add the person to have their "ban report" removed, or else their account will be "banned".
Please, for the sake of keeping your games / virtual hats / what not, do not click the link they send you, or any suspicious link, for that matter. It is most likely a redirect to a website looking like the usual Steam Community website, except loaded with malware and all that gnarly stuff to steal your account information and who knows what else.
Block them if they appear, gather evidence, report them, etc. if you see them.
Stay safe out there.