〜で details
in total; as a group of — で on a quantity bundles it into a single unit
昨日、三人でカラオケ行ってきたよ。
kinou, sannin de karaoke itte kita yo.
The three of us went to karaoke yesterday. casual
お会計、全部で三千円になります。
o kaikei, zenbu de sanzen'en ni narimasu.
That comes to three thousand yen in total. neutral
Examples
as a group of — the participants act as one unit of that size, one included (一人[ひとり]で = by oneself)
昨日、三人でカラオケ行ってきたよ。
kinou, sannin de karaoke itte kita yo.
The three of us went to karaoke yesterday. casual
引っ越しの片付けは一人でやりました。
hikkoshi no katazuke wa hitori de yarimashita.
I did the unpacking after the move on my own. neutral
in total — the separate amounts added up and stated as one figure
お会計、全部で三千円になります。
o kaikei, zenbu de sanzen'en ni narimasu.
That comes to three thousand yen in total. neutral
こちらの資料は全部で十ページございます。
kochira no shiryou wa zenbu de juppeeji gozaimasu.
These materials run to ten pages in all. formal
Formation
{N}でattaches to noun数＋助数詞＋で（三人で・三つで・全員で）attaches to noun phrase (schematic)
Related patterns
Similar
Both put a number in adverbial position and they point opposite ways — で bundles the quantity into one acting or totalling unit (三人で分ける = the three of them split it between them), ずつ hands the same number out per recipient (三つずつ分ける = three apiece).
Lookalikes
Same で, and what precedes it decides the job — a place or a tool noun hands the verb a venue or an instrument (電車で行く), while a quantity word bundles the participants or the sum into one unit, so 三人で行く is "the three of us went", never "went by three people".
Same numeral, and the particle carries the whole claim — 三人で来た says they arrived as a party of three, 三人も来た says three came and that was more than the speaker bargained for.