For parties and events, prioritize a wide maximum aperture over a huge zoom range. Indoors, an f/2.8 zoom is much more useful than a slower superzoom, because it helps keep ISO lower and avoids relying on flash.

From the suggestions given, the Tamron 17-50mm f/2.8 is a strong budget match for a Canon 600D. It’s affordable, reasonably sharp, and the 17-50mm range works well on APS-C for groups, general candids, and close event coverage. The non-VC version is often considered a bit sharper, while the VC version adds stabilization.

Another option mentioned is the Sigma 50-150mm f/2.8, which can work well for candid shots from farther away, but it may be too tight indoors unless you also have something wider.

If budget is very tight, a fast prime like the 50mm f/1.8 is a cheap way to get better low-light performance, though you lose zoom flexibility. An external TTL-compatible flash is another practical solution for dark venues.

In short: for a low-cost third-party zoom, the Tamron 17-50mm f/2.8 is the safest recommendation here.