Once you're inside the group and approved as a member, click on Members, then filter the list of members further by searching for certain job titles or something else to winnow down the list to your ideal prospects, and then invite them to join your group (tip: send personalized invitations). Once these invitees join your LinkedIn group, you have all your proverbial fish in the same barrel — all your best prospects in one place! You can control this LinkedIn group so that no competitors get in, and you can share great/valuable content within the group that your prospects will love.
You also get to demonstrate your value/expertise for them while avoiding overt sales pitches or spam. Plus, you also have a built-in email list, focus group of your core prospects/clients and so on. This is a great tactic to build your brand and generate leads to boost your small business.