From the aspects I have read here and on the Sims3 site, the Simport does seem totally optional. My rough guess is to control who can send a Sim to your town to perform, each game may come with a type of friend code, which would be needed to be used to add the friends that can come to your town. Using a shortened version, person A has friend code AAA and person B has friend code BBB. they would have to exchange friend codes to be able to send Sims to each other's towns. When person A wants to send their Sim to person B's town, a pop up will appear in person B's game asking for permission. If person B says yes, then person A's Sim will go to that town and perform. I would have to double check, but I think person can que up actions for the Sim's performance, either during or before, while person B watches. After the performance, person A's Sim goes back home with whatever the last item thrown on stage is.
That is roughly how I understand how Simport is going to work, along with my speculations. Friend codes just seem to make the most since to me. It's either that, or its connected to your friends list on the official site, which could also make since, seeing as that is where you can upload memories to, as well as where achievements are probably recorded.
And those achievements do sound fun. I look forward to seeing what, if any, rewards there are for completing them.
