A lot of groups have “promo days” where you can openly promote your website, products and services. If a group has promo days then the owner will have a list of which days promotions are allowed so you need to find this.
When you are a member of several different groups then we recommend that you create a spreadsheet of the promo days for each of these. If you are in enough groups then you will find that you can promote your stuff several times each week.
Once you have established yourself in the groups then use the 80/20 rule for promotion and content creation. You want to organize it so that you spend 80% of your time on promoting your content and 20% of your time on new content creation. This will help you to generate the most free traffic.