Joining a Retweet Group
Let's say I'm an author and I make a post to promote one of my books. The post looks like this.

Some of those hashtags make general sense. The #FREE, #FreeEbook, and #FreeBook catch the attention of people out there looking for free stuff. The #YoungAdult, #Dystopian, and #Western categories draw in readers of those genres.
But what is #CR4U and #IARTG?
Both of those are writer-based retweet groups. #CR4U is a group specifically focused on clean (non-explicit, non-sexual) fiction. #IARTG is a more generic group that is about any independent authors at all. I have joined both groups and participate in their retweet system.
In general, the group members watch over the hashtags associated with the group. So the #CR4U group members keep an eye out for that hashtag. When they see something come through with it, they retweet the post. That means, when I make a post using the #CR4U and #IARTG tags, I can expect to get 10-50 retweets of my post. That then reaches thousands of new eyeballs.
When I see that someone has retweeted one of my posts, I then go to their page and retweet one of their book promotions. So we help each other out.
This type of retweet group is 'voluntary'. A member doesn't have to retweet every single post they see. If an erotic author happens to post something explicit, and I don't feel comfortable retweeting it, then I don't. It is intended to have everyone help each other out as they can.
Mandatory Retweet Groups
There are also groups, often organized on Facebook or other systems, where there is a mandatory reciprocation of all retweets. That is, let's say a group sets up a thread for Monday, April 1st. Each author makes a tweet promoting their book and posts the URL of that tweet in the thread. Now every other author is required to retweet that post.
On one hand, this enforces full participation. If there are 50 members of the group you know for sure that your post will get 50 retweets. It's a comforting feeling.
On the other hand, this means you as a participant are forced to flood your thread with 50 posts about other peoples' books even if you don't agree with them all. Even if one has explicit content that you're unhappy with. Even if one involves anti-black or anti-gay comments you disagree with. This is your own thread that you're promoting the items on. It can become sensitive.
So my suggestion in general is to participate in as many voluntary groups as you can find. They're a great way to spread the news about your projects and it lets you choose who to promote. I'd generally shy away from the forced-reciprocation groups as potentially causing issues.
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