Translation:We employ people who have a lot of experience.
"big" doesn't work with "experience", but you have a large range of other options. You can say "We employ people who have a lot of/wide/much/a wide (or large/broad) range of/broad/extensive experience". Note that all of those except "a lot of" and "much" imply experience in a variety of things, not just quantity. For the opposite meaning you would probably use "little" or the less common "scant" (which I prefer myself).
OK, I see the pettern here, I see that mając/robiąc etc are declining, I see that here mając has declined to genitive plural, presumably to match the plural "ludzi"
But I still don't see why?
My head is remembering a previous exercise where the question was something like "Chcę jeść coś innego" and I'm guessing that "mając" is declining for a similar reason that "inny" declines into genitive singular to match "coś"?
Actually, 'mając', 'robiąc' and similar are a different thing, those are adverbial participles, which mean 'while having', 'while doing', etc. Those do not undergo declension.
'mający', 'robiący' and similar are adjectival participles (The man who has a car, The boy making a sandwich) and those do undergo declension, exactly the same way that normal adjectives do.
And "coś innego" is in general quite a strange thing, a noun phrase "coś + adjective" always takes an adjective in Genitive, for reasons that I absolutely don't know. Of course this adjective can later undergo declension (e.g. "Porozmawiajmy o czymś innym" = "Let's talk about something different), but if we just look at the noun phrase on its own, without a sentence, it takes Genitive.
alik1989, Jellei, once again thanks for your help...
This topic has been quite challenging, I think the only one that I've found more difficult was Polish numbers, which I still struggle to find the patterns in...
Between your two replies here I understand the concept of this topic, and the difference between the adverbial and adjectival participles
Let's leave "coś innego" alone for now, it seems to ask more questions than it answers :D