Somebody is following us (passive

We are being followed by someone.

To put a sentence into passive voice, we make the object of the original (active voice) sentence into the subject of the new sentence (in passive voice).
“Us” is the object in the original sentence, so we change it to “we” to make it the subject of the new sentence.

Then, we keep the verb in the same tense. So we have “We are ...”

Now we change “someone is following” into “being followed by”.

Finally we put everything together: “We are being followed by someone.”