There is a “subscribers” in the member profile, but I have no idea how to subscribe to a member
If you go to their profile (for example, you can click on my name to go to my profile), you will see a “subscribe” there.
If this doesn’t help, let me know! 
I see, thanks! - in my friend’s case instead of “Subscribe” there’s a “Pending” - what does this mean?
It means you have already requested to subscribe but they still need to approve.
I see, thanks! - I’ll send her a message
You are quite welcome.