question about missions

I was doing the Vizunah Square mission on Factions and I noticed that there was another player's party within the mission. I normally play solo, so I was surprised to see another team. I admit his team helped out as there were many afflicted to fight.

I have 3 questions about this:

1. How were we able to see each other when neither of us accepted any invitation from the other?

2. How can I do that again? Is it simply a timing thing where we need to enter the mission at the same time?

3. Does this work for all missions for each campaign? or only certain ones?

There are two missions in factions that function this way. The party's are made randomly and there is no way to decide with whom you play the mission. You both start at a different outpost. In this mission it is as follows.

Characters from factions follow their questline to Vizunah square local quarters.
Characters from Nightfall or prophecy's follow their questline to Vizunah square foreign quarters. The mixing is done at the start of the mission (therefor the starting countdown can be 60 seconds long instead of the normal 10 seconds). The first part of the mission each follows different objectives.

Local quarters (so people from factions) follow master Togo and defend him at the center. Foreign quarters follow mehnlo and fight their way to the rescue of master Togo and the local players. After that the mission is the same for both party's.

You are not able to see the other party's life-signs and healthbars but skills affecting allies work on them as well.

You can talk to them in partychat or local chat to say hi or discuss tactics (for example wich side to defend in the different parts where you have to fight several waves of afflicted.

The other party is selected from the same district, so you prolly speak the same language. if no other party is available you get assigned to a party consisting of henchman (wich makes the mission a lot harder in most situations).

It is possible to team up with friends by what is called sync jumping. in that case you synchronise entry by using whispering or teamspeak/vent. in most cases you'll end up with eachother.

There is one other mission like this called unwakening waters. although groups and starting outpost are determend different I'm not going to explain that cause it would mean spoiling part of the story.

As above - it's a normal part of that mission.

When you enter the mission, there is a countdown during which the game waits for a team to enter from the other Quarter. If there is a team, the countdown can be shorter.

If no other team enters before the countdown ends, then the other team will be made entirely of random henchmen.

It is sort of possible to co-ordinate this. If you know another team/player who wants to do the other Quarter at the same time, you can co-ordinate your timing by chat or Ventrilo, etc, to enter at the same time. That used to be difficult because there were a lot of teams entering and the choices were basically random, but these days, you have a better chance because the game will take human players over all-henchmen teams.

You don't have to enter at the same time. As long as the other person enters before the timer reaches zero. And yes, due to it's lack of activity it's really easy to team up with your friends.

I really doubt this. Probably your district gets priority, but it's not a necessity. I never ever had a party that would speak my language on the other side, and there have been of many different languages (in fact, I hardly ever managed to communicate succesfully with them).
But nowadays you mostly get hench team so...

Most of the people I've run into in the two missions have spoken the same language as my native lang (english). Sometimes I've wished they wouldn't, but I haven't run into the case where they spoke something else much.

I've run into cases where it actually would have been easier if they hadn't been there.