Here are a couple videos that explains exactly what power creep is on both offense and defense. In short, it simply means when new content makes older ones underpowered
In Pokemon terms, it refers to mechanics, changes, and new Pokemon taht changes the game. Some mechanics would involve stuff like the generational gimmicks and changes would be about the weather ability nerf. The term 'group' is inaccurate as often times, this is not always the case. For example, in gen five ou, two of the four swordsmen were heavily influential in the tier while another lone case like Reuniclus was also influential. It's hard to nitpick these across the generations but you can often see this by looking up the tier's viability rankings across the generations and seeing which previous mons have dropped or risen through the tiers. There are also Pokemon who are always staples in the tier they're because they're so robust. Things like Garchomp, Heatran, Ferrothorn and Landorus to name a few
Your last question is kinda difficult to answer because each generation brings with them power creep that makes Pokemon faster, stronger and bulkier. Some of the most notorious power creep would have to include gen five because of all the crazy stuff it brought. Kyogre and Groudon passing their abilities to their followers, gems that turn high damage moves into nukes, hidden abilities, the unkillable jellyfish in rain, the swordsmen, among other things. Gen seven would also count since gen six already upped the level of power with the megas but gen seven took the gems concept from gen five and made them into even more of nukes in the form of z moves. Of course, gen nine would also count given the recovery moves nerf and the ability to change type. The fact that a quickban happened like two days after gen nine was released should be a testament to this thing's power creep
Either way, those videos I linked earlier would explain power creep in Pokemon in great detail, much moreso than I can
