Etiquettes

Greeting with salaam alaikum

Q: Is it ok to just say سلام عليكم? In Hayaatus Sahaabah (Part 2), under chapter 9 -

خروج الصحابة رضي الله عنهم من الشهوات - :التسليم على المسلم - عمل ابي امامة رضي الله عنه في ذالك

A riwaayah comes wherein Abu Umaamah رضي الله عنه said this.

Not making salaam

Q: To make Salam to others is sunnah. If a person does not ever make salam to others, is he a sinner? Can we blame him?

If a group of people come to another group and nobody in the group does Salam, will they be at fault?