I think it probably means something like "getting one good time". It doesn't make much sense in english, but it might make sense in japanese.
In the Japanese dub, typically "Get" is the word used for "Catch". (e.g. "Pokemon getto-daze!" literally means "Get Pokemon!", but it's transliterated as "Gotta catch Pokemon!" This is where the "Gotta catch'em all!" tagline originated from).
It could just be a normal use of "get", but given the precedent, by default it's somewhat implied they're talking about a catching a Pokemon of some sort. To be honest, I am hoping Ash and Serena's relationship is more in focus this episode, rather than just a capture that just takes place at a party where the two happened to be dancing.
If both Ash and Serena end up each catching a Pokemon in the same episode that'd be pretty cool, though. It's never been done in the anime before for Ash and the female lead (Ash and Cilan caught Palpitoad and Stunfisk at the same time, but I'm sure this one will be handled better). Also worth noting that Palpitoad was the only Pokemon nobody knew Ash would catch in BW - it came completely out of left field.
On a side note, this sort of subverts a theory I had. I thought they might use Showcases to develop Ash and Serena's relationship by requiring a Pokemon Performer have a human partner with them in one of the upcoming Showcases, so technically Ash could get a closer look at what she does and be involved in her performance.
This episode might either imply that sort of idea's already being used (the two dancing), or that it might just be similar/pave the way for the idea (if it ends up happening).
@Date episode - Well, it wasn't an outright date, but many suspected it wouldn't be. It was the first instance of Ash giving a female a gift without being coaxed into it, though. And the ribbon was later used by Serena in her new outfit as part of her being inspired by Ash, which was a pretty cool touch. While Amourshipping is often kept at the side of things, they're clearly just developing it slowly. Amourshippers will likely appreciate any reasonable developments, though obviously they'd like more substantive ones. Whether or not things will reach a climax is unclear since Serena's feelings for Ash are unique amongst female leads.