My computer specs are: Intel Celeron processor 1.40 GHz, 303 MB RAM, Mobile Intel 915 GM/GMS, 910GML Express Chipset Family.
Ah, OK. Sadly, the problem lies within your computer, as it doesn't quite meet the minimum requirements for AL (your latest EP). I suspected it might be problem, though I was hoping it was a less expensive fix (like a mod interfering with your game). Your game will only allow you to invite guests based upon your computer specs, particularly your processor.
Here are the minimum requirements (taken from the Sims 2 site):
1.8 Ghz processor (2.4 for Vista)
512 MB RAM (1GB for Vista)
Windows(R) Vista, Windows XP, Windows ME, Windows 98 or Windows 2000 Operating System.
8x or faster CD/DVD drive
At least 1.5 GB of additional hard drive space (5.0 GB total is required if installing both The Sims 2 and The Sims 2 Apartment Life at the same time.)
A T&L-capable video card with at least 32 MB of video RAM.