Rashi: It is a group.

Radak: It is like when the three root letters are reversed, i.e. Koheles (assembly). Malbim - we find roots that can reversed, e.g. Keves - Kesev, Simlah - Salmah. Lahakas is the entire group; Chevel [Nevi'im] is some of them.

Radak citing Targum Yonasan: It is a company of Safraya, i.e. Chachamim.

Radak citing Targum Yonasan: They were praising, i.e. saying songs and praises to Hashem with Ru'ach ha'Kodesh.

Radak: Perhaps they were saying prophecies that will occur soon.

Radak, Targum Yonasan: He was standing on his feet and teaching them. This is like "ha'Nitzav Al ha'Kotzerim" (Rus 2:5) - he is appointed over them to show them what to do. Also "Nitzav Al Avdei Sha'ul" (22:9) is the greatest appointee over them.

Malbim: Amidah is mere standing. Nitzav is Amidah that requires strength to retain one's position, e.g. in front of a great person, or one's opponent. It is also one who is appointed over and oversees others - "ha'Nitzav Al ha'Kotzerim" (Rus 2:5). All other Nevi'im, at the time of Nevu'ah, they fell on their faces and their limbs falter. Only Shmuel was different. (a) He remained standing. (b) He was Nitzav Aleihem, and put an influence of Nevu'ah on them. He was the source from which they drew, the lamp from which their souls were lit with Divine light.

Malbim: The influence was complete due to (a) the influencer (Shmuel); (b) the receivers, i.e. the entire group of Nevi'im; Kedushah is greater among a large group; (c) the connection between the source and the recipients. This lit the sparks of Kedushah in the souls of the messengers, and they had a low level of Nevu'ah. Amidst this, they did not heed the command to take David.