This is a simple guide for your Social Club settings.
Step 1) Go to your R*SC profile, click the little cog in the top right corner, and go to 'Settings'
Step 2) Click the 'Privacy'-tab
Step 3) Set your profile and linked accounts visibility to "Friends & Crews" (or everyone)
NOTE: You do not have to share ALL your linked accounts. If you want to keep your facebook/twitter/google etc. private, you can. But your PSN/XBL/Steam must be shared.
If you're accessing R*SC on mobile, make sure you're using the desktop-version in your browser, or use a regular computer/laptop. The mobile version has limited functionality and might not show you all options. If you still don't get the option, it's most likely because you're not 18 yet, so R* won't let you share (privacy blahblah) In that case, send one of the Comm's for your platform a message, and it'll get sorted.
If this is the case, you are most likely <18 of age. Rockstar appearantly blocked sharing for minors for privacy reasons. Send a friendrequest on SC to one of the Commissioners of your specific crew, and a PM it's for promotion, and it'll be sorted eventually.