Use the following cheat in the console or the userstartup.cheat file:
maxnumofvisitingsims 20
Replace 20 with the wanted number. Large numbers will cause your game to go splodie unless you have a Cray.
You can invite all playable Sims if you have a mailbox on a lot and shift-click it with 'invite all neighbours'.
Sorry to correct you Jordi but the right line is:
intprop maxnumofvisitingsims 20
I am surprise nobody notice that