You can enter the campaign solo and even if u have loading screen u will end up in the same campaign, given that you entered the same named campaign. You can group up before or after joining a campaign, it really doesn´t matter. Just wait for the loading screen and your friend will be there waiting when you arrive.

Unfortunately nowadays you get a the pve buffs (keeps, emperor etc) in pve and that makes some alliances own an entire campaign. This is not productive pvp, Azura is blue for example. And people tire of trying to take things back cause of endless numbers of blues defending their blue campaign. Same thing goes on in most campaigns more or less. Thornblade seems the one less dominated by one alliance. August 31st I hope they fix this, since pvp buffs will no longer be available in pvp.
Hope you have a good pvp experience nonetheless and maybe try out some other pvp campaigns. You can have one home and one as guest and up to august 31st you can port to any player in another campaign.