Learn Finnish: Odottakaa täällä seurueinenne. - Please wait here with your entourages.

It comes from the noun seurue (entourage or party). To form the comitative case, we add the marker -ine- to the plural stem, followed by the possessive suffix -nne (your, plural). So, seurue + ine + nne becomes seurueinenne.

Does seurueinenne mean one entourage or multiple?

It can actually mean either! The comitative case always takes a plural form (using the plural stem before the -ine- marker) regardless of whether the meaning is singular or plural. You have to rely on context to know which one is meant.

Could I just say seurueine without the -nne at the end?

No, when you use the comitative case with a noun, Finnish grammar strictly requires you to attach a possessive suffix to the end of it. Because the command is directed at a group (you plural), we use the corresponding suffix -nne.

Why is the verb odottakaa instead of just odota?

Odottakaa is the imperative (command) form for te (you, plural or formal). The ending -kaa tells a group of people to do something. If you were telling just one person informally to wait, you would use odota.