Just jump in. Most groups are moderated. You can search on line and find some of the more popular or more useful to budding writers. I am happy with what I have experienced. Overall I’ve found most reviews/critiques I’ve read to be helpful, offer constructive ideas and suggestions for improvement, and most importantly–the reviewers are polite! I’ve spent time reading others’ reviews (probably as much time reading critiques as reading stories!)
I’ve also critiqued. At first it was difficult and I am not sure how helpful my initial critiques were. I am gaining confidence and I feel my reviews are improving. One hindrance is that I don’t feel I am particularly good with grammar. I can read a sentence and know instinctively if something ‘sounds funny’ but I also miss a lot of the finer points. I don’t always know where a comma or semi-colon should be entered. I get my tenses mixed up sometimes. But hey, guess what–even though I am not well versed in the intricacies of the english language I am a reader.
I ‘know’ when something doesn’t sit right. I feel it when there’s something missing from a plot. I get confused when things don’t make sense 
I can contribute. So, that’s what I’ve been doing. So far so good. Maybe the people receiving my reviews and suggestions have just been plain nice people…or maybe, as a reader, I DO have something to contribute.
It is a new adventure. And, yes, it’s into the unknown. But, so far (at least for me) it’s been working. If you are a budding writer you might want to consider joining an on-line critique group. You will have to commit to critiquing others’ work yourself–but I can guarantee you will learn from the experience and your writing will benefit from it.