If you're looking for specifically what the last fireteam you played with was for Trials, we do not have an endpoint that directly provides that. What we do have is "PGCRs" - Post-game carnage reports. These are the full details of individual games, and through it you can infer Fireteam data and stats. The DestinyTrialsReport team does thorough PGCR inspection in order to produce their data: it's been a pretty monumental undertaking for them, as in order to get that data you need those PGCR-level details pivoted by character rather than by game. Essentially they had to (and continue to have to) process every game that passes through the system. You should speak with them, they might be interested in joining forces on such a project: and if so, you could leverage what I imagine must be an absolutely massive pivoted dataset.