Are Ainz and his servants still leveling up the same way they were when they were killing monster to get exp in Yggdrasil?
Hmmm... To my understanding, Ainz and the high-tier minions (read those that are level 100) don't level up any further, as they are maximum level already. It's never mentioned that they 'get stronger' while others like the lizardmen and that weird hamster indeed do (the narrative comments several times that they are training to get stronger). Now, it is possible the New World has a higher level cap but insofar it hasn't been mentioned at all in the nine LN, the WNs nor the anime and manga. So Ainz ain't levelling nor are the floor guardians, but some of his minions do.
Lvl 100 was maxed level in the game, but the New world has different laws and something from the game manifest differently. This is because the new world isn't a game, but instead, an actual world that is trying to make sense of the game mechanics.
In the game, the minimum amount of experience a monster could give is 1 and also Ainz had 100% experience to level if the game allowed him to become level 101.
Ainz should have leveled already if he was gaining experience points, the only caveat is that what if the New world is giving humans or other monsters less experience. For example, if the new world gives humans 0.001 experience then it could still be possible, but there isn't anything that suggests that Ainz can level.
Last note, the author himself said Ainz is maxed level and he won't be able to level.
Reference to the translation, which has a reference to the original.