Perspective:
Religion is a way of life as we are told, that is not extreme.
You are finding it extreme that they are advising someone who was seemingly interesed in religion by signing up for the associated society according to the very principles of that society? I find that 'weird' to say the least.
I am by no means judging you, merely picking out a flaw in the logic of the first post.
If you don't want any advice or part of the society, leave any WhatsApp, Facebook etc groups and tell them you don't want to be a part of the society, but do not try to tar them with the same brush and label them 'extreme'.
Remember free mixing is not allowed, alcohol is not allowed etc. This is not them being 'extreme' as you put it, its called having integrity and not a hypocrite to what they claim to believe.
Simply saying do not hang out with non-Muslims sounds bad, but in the context it seems they are simply trying to help you avoid any potential situations which would lead to you doing things contrary to the religion. You can make the ultimate decision of your company, but it is not 'extreme'. It is similar to any advice you get from your parents to not hang out with criminals, why? Because it may lead you to potential situations which would be contary to the law, for which you could lead yourself to trouble and be repramanded.
PS. Coming on TSR with this stuff is not a good idea. TSR already suffers from people who cherry pick quotes and generalise all Muslims and this sort of stuff only adds to their views which are merely a figment of their imagination.
I have outlined the facts, given you some advice for your situation and only asked of you to present the facts objectively. I hope I have not offended you and I hope your studies and views of your fellow Muslims is not damaged by any of this.
I have no doubt that an atheist will take my post out of context so my advice to anyone thinking of doing so, save yourself a few minutes and enjoy your life, for as you put it - you only live once, right?
All the best OP.