Go for it as far as i know most Railway's will let you join on the day,most have membership forms at the booking halls and offices,but i always find that the more you put into it the more you'll get out of it
The ELR is great and i think i would join as they have a few BR diesels but i live a fair way away so would i get what i want out of it sadly i dont think i would
I look forward to your post's about it
All the best,Ian