@nildarar, if they mention their friends correctly–via @ tagging–then their friends receive individual notifications directing them to the topic (as you should have received for this reply).
Additionally, if users have gained the right trust level, they can invite other users to topics.
Red = invite notice
Blue = by whom
Yellow = where (topic/post)
Purple = when
This is offered organically for every topic via the OP, once there has been a like or reply, by selecting the down arrow.
I agree with @Paracelsus that a count of “currently viewing” for each topic might be a nifty option.
Otherwise, anything else that publicly specifies to other users what topics a specific user has viewed (because nothing guarantees read) just seems nosy at best and potentially exploitative at worst.
ETA: To be clear–just because User1 wants User2 to read their topic does not entitle User1 to any indication of whether User2 viewed the topic or not. Whatever is happening there might be more appropriate for a PM.