May one count before Maariv?
It is permitted to count the Sefira before Maariv [after nightfall]. This applies even on Motzei Shabbos, when one extends the Shabbos into the night [until Maariv]. Nevertheless it is proper to precede the Davening of Maariv to the counting of the Omer.
Is it proper to count the Omer together with a Minyan?
Yes. It is proper to count the Sefira together with the Minyan. [Thus one should not leave the Minyan for Maariv until after Sefira.]
If the Minyan is holding by Sefira and one has not yet begun Maariv is he to recite Sefira with the Tzibur?
 Admur ibid; M”A 489/7
The reason: As nevertheless it is already night and is not considered the previous day but rather the next day. [Admur ibid]
 Admur ibid; Chok Yaakov 489/16
The reason: As Maariv is Tadir [i.e. common] while the counting of the Omer is only on occasion, and the rule follows that a more common Mitzvah receives precedence over a less common Mitzvah. [Admur ibid]
 Shelah brought in Chok Yaakov 489/16 “Sefira with a Minyan”; Siddur Yaavetz “Sefira with Rov Am”; Minchas Yitzchak 9/56
 Minchas Yitzchak 9/56
 The reason: As it is proper to count the Omer with a Minyan [Poskim ibid] and one should not separate from the Tzibur. [ibid]