We normally prefer to discuss account suspensions like this in private, but because there might be broader points to be made here, I'll provide a little more detail.
We have many, many users here who share the same outward-facing IP address as others, as well as a lot of coworkers and friends who all use the sites at the same time. It is perfectly fine to have multiple people from the same place asking and answering questions here, but there are some activities that throw up red flags and cause us to take a closer look.
Unfortunately, we've had problems with groups of friends and coworkers who have attempted to manipulate the voting system by all agreeing to vote for one another's posts. Others have created outright fake accounts or had friends create accounts just to vote for their content. This can lead to terrible questions or outright wrong answers being voted higher than much better ones, reducing the quality of the content here and undermining the trust people have in the Stack Exchange system. As a result, we are somewhat suspicious of excessive numbers of votes being exchanged between people at the same location.
Without going into specifics, we have tools that notify us of anomalous voting patterns like this, as well as observant users that flag odd activity. In your case, that happened multiple times across multiple Stack Exchange sites, which is what led to the removal of your friends' accounts.
The problems in your case started on English Language and Usage, where you and two other users were found to be coordinating votes for one another. Every single one of your four answers there were to questions by one of two users who shared an IP address with you, and they received an abnormal amount of votes given the quality of the answers, which set off warnings among the users there. In particular, this answer being accepted by your friend above others of better quality looked extremely suspicious. This led to the suspension of your account, those of your friends, and the manual invalidation of votes between the three of you.
You then proceeded to do the same on Stack Overflow. Your two friends only created one question and one answer each, which you apparently voted for, then they cast votes exclusively for you. Because your friends didn't provide much of value in those accounts on Stack Overflow, the easiest thing for us to do was to destroy these new accounts. Despite this, one of your friends created a new account and used the 100 point account association bonus to get them to the level where they could vote for you again, which they proceeded to do. We again destroyed this account, because it had nothing of value to it and it was being used to manipulate the voting system.
Votes should come based on the quality of an individual question or answer, not because of who is behind it. While we understand that you're more likely to look favorably upon a coworker's posts, please do not go out of your way to vote for everything they write. When the only posts that you vote for are those by people from the same location as you, that looks extremely suspicious. If you and your friends refrain from this behavior in the future, you all are perfectly welcome to have separate accounts and keep posting questions and answers here.