Lets say the shul is being mekabel early Shabbos. If they already said Mizmor Shir L’Yom Hashabbos, the tzibbor is considered to have been mekabel Shabbos. So if someone walks in now, they cannot daven mincha in the shul, even though it might still be an hour to shkiah.
This does not apply once the minyan is done and has left.
This is important to know during the summer months when many, if not most bring in the Shabbos early.