What kind of posts do you make? These groups are mostly allergic to anything they perceive as self-promotion.
First thing I do when scoping out new groups - not so much anymore, as I have twelve or fifteen with reasonably good membership - is to look at the rules. These rules have a little icon when you go to post. Some ban self-promotion altogether unless asked. Most of the ones I participate in have a 1 self-promo per week rule. This can get a little tricky - usually they pop up right away, but many times the post won't get approved for several days after I hit the 'post' button. I go by those dates.
Typically, I make my posts to all the sites on the same day. About half of these will be for the book I'm pushing at the time - yesterday it was 'Empire: Metropolis' (book 4). The other half gets split - yesterday, the split was between 'Empire: The Complete Collection' and 'Empire: Country' (book 1).
As to the posts themselves, what I've done is make up a generic plug for each book, including the link. I then do a straight copy-paste, putting the title in bold. This one is typical. (I intend to track down more artwork for variety.)
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EMPIRE: THE COMPLETE COLLECTION
Like Lovecraft meets Game of Thrones!
Tia Samos sets out into the tottering Solarian Empire intent on finding a highborn husband. She is accompanied by her maid and personal minstrel Rebecca, her knightly protector Sir Peter Cortez, and Kyle, a veteran with a knack for magic, searching for a place in a nation they no longer recognize. What they find is opportunity, peril, forbidden secrets…and sheer terror.
The Complete Collection incudes –
Empire: Country
Empire: Capital
Empire: Estate
Empire: Metropolis
Empire: Spiral
Empire: Judgment
Available now at your favorite digital store!
Empire: The Complete Collection by Tim Goff
books2read.com