Maintenance for the week of November 18:
• PC/Mac: No maintenance – November 18
• ESO Store and Account System for maintenance – November 19, 9:00AM EST (14:00 UTC) - 6:00PM EST (23:00 UTC)
• PlayStation®: EU megaserver for maintenance – November 19, 23:00 UTC (6:00PM EST) - November 20, 17:00 UTC (12:00PM EST) https://forums.elderscrollsonline.com/en/discussion/668861
Can you join the same campaign you have as a home campaign in a different alliance. Ex/ If I join the first campaign as a DC, can I have another character in EP or AD in that same campaign?
No you can not. Each campaign is Alliance exclusive for you account.
You set Auriel's Bow as your home, for instance, on an AD character, only your AD characters can set that campaign as home as well. You will have to choose a different home campaign for your EP and DC characters.
How ever I do not know if it will let you guest on that server with non-alliance characters. Maybe someone else can provide further clarification.