Why did he call them Bnei Yemini?
Radak: You are from my Shevet and family - you should have been concerned for me!
Malbim: If Yehudah are supporting David, it is because they are his Shevet and family. They hope that he will [reign and give to them] authority or an inheritance of a field or vineyard.
Why did he say "Gam l'Chulchem"?
Malbim: Even if you hope [to get authority or an inheritance from David, all of you cannot be officers over 1,000, and you cannot all receive fields and vineyards. 1
Shmuel I 8:14 authorizes the king to take people's fields and vineyards for his servants. R. Yosi (Sanhedrin 20b) holds that the king is truly entitled to this! Radak there said that he may take the Peros, but not the land itself. (PF)