Usually when there isn't an option to Invite to Current Lot, it means that sim is already nearby. Click on the sim you want to invite and choose Focus Camera On. The camera will pan to that sim. You can then click on the for your sim to interact with. I usually put them in a group so I can make the other sim follow my sim home.
Edit: I just realized I may not have been clear enough. In your sim's friends list, find the sim you want to invite over. If there isn't an option, click focus camera on and it will will pan to that sim.