Tell them you just want to check the exact weight and balance and performance graphs before inviting a couple of friends. So many PPL holders (even experienced ones and even CPL holders who should know better) fail to check the W&B before a high-load flight you should impress them not make them suspiscious!
I know of a very experienced pilot (same squadron as my father in the late 60s, so very experienced) who was seen to try taking off in a PA-28 with 4 on board on a short grass field. I had previously done the calculation for a similar aircraft, similar field, 3-up, mid fuel, and that was marginal! He failed on at least two different runways, trying to get the best wind/length combination and in the process churning the slightly muddy ground with heavy braking before narrowly clearing the hedge. If you read GASIL (UK General Aviation Safety Information Leaflet - sketch outline of recent accidents and causes) you will always read of aircraft trying to taking off outside the limits with less fortunate results. It is good to see you are not likely to be following them!