I had the same issue Marty. I use a 2016 version of Slimjet, so it's probably a browser issue, as BTBS mentioned.
If I right click on a player's name and select open in a new tab, all of the buttons show up. If I just click their name, nothing shows but add friend, even if they're already a friend.
Try the right click on a player's name...see if it works.
If not, until it's fixed (if it's a WGT-side issue), you can invite using the mobile version (or an emulator if you don't have mobile app access).