"I create the collection , then go to grid view and click Share in the top right hand corner. HOWEVER, the "share" button has dissapeared. How do I share a collection from lightroom classic?"
After creating a collection in LR Classic you need to click on the double-arrow icon to the left of the collection name. This will Sync the collection to the Web. Once you've done that you will see a 'Make Public' button in the upper right-hand corner of the Grid view along with the share link URL.
A better way to do this is to access the synced collection album at lightroom.adobe.com and click on the face icon in the upper right-hand corner. You can then access and set specific settings as to what your clients can see and do.