You can be forgiven for not remembering the details about Oceanside.
The Walking Dead only visited this all-female colony once, way back in November, in what was at the time the least-watched new episode of The Walking Dead since Season 3 (it's since been replaced by "Say Yes").
In that episode, "Swear," Tara (Alanna Masterson) washed up on Oceanside's shore after getting separated from the still-missing Heath (Corey Hawkins). She learned that this community used to be under Negan's (Jeffrey Dean Morgan) rule in another location, but after the Saviors killed all their men and boys older than 10, they fled in the dead of night and ended up here. The Saviors don't know they're there, and Oceanside's leader Natania (Deborah May) wants to keep it that way. They want to be left alone, and they protect themselves with a substantial arsenal.
So now, Tara is leading the Alexandrians back to Oceanside, where everyone wants her dead except for one person, who will now think she's a snitch.