To me, it seems that Dawnguard is more of an organisation than a guild. What are your guys thoughts? On the other hand, itt's not ''lore friendly'' to be in two guilds at the same time so it would be weird to be in Dawnguard and Companions at the same time etc..

Well I think guild and organization are two fairly interchangeable terms since a guild is a group of people pursuing a common goal and an organization is, well, pretty much the same thing. On the other hand, in terms of the relationship of guilds in TES, I'd say Dawnguard is not a guild. In TES guilds are groups of similar people (assassins, thieves, warriors, mages) and Dawnguard is a group of all kinds of people that was formed with a specific goal in mind, i.e the extermination of vampires. Looked at this way, I'd say Dawnguard does not belong in the same category as the guilds.

To me I treated the entire expansion as an introduction to a new guild as it followed the same structure.

Join them, do some radiants, do the main quests, the MQ was probably a bit longer than a guild MQ to me when going strictly for objectives, you get a new base in the end of the storyline, become head of the group at the end, so on and so forth.

To me it was essentially just, guild stuff.

I was about to join the vampires but they were a bit.. psycho in my opinion. I mean, being extremely evil is cool i guess but they have freakin human parts everywhere in that castle. To me it seems like they are canibals rather than vampires xD

You mean its not narrow minded player friendly. NOTHING in lore says one can't belong to multiple guilds. Don't confuse them with morrowind's great houses.

My first character joined so many guilds and organisations one more was not going to make any difference. I just thought of the Dawnguard as a guild of specially organised vampire hunters.

Its not ?? never heard of that. You are Saying that an Assassin Mage cannot be in the DB and in the Mages Guild? DB at night to kill and MAges guild by day so that people trust him.
i think the dawnguard is more an organization guild.

Yeah might be true, but I think we both can agree that it's more realistic to be in one guild However, my character is a Companion (harbringer rank) and decides to take sides with Dawnguard to solve the vampire menance. But his main faction is always going to be the companions.

I'm doing a TG/Dawnguard Bosmer character. I don't find it odd to do both in the way that I would find it odd to make a character that does the Companions and the DB questlines together say.