While I havent had problem, I have found on numerouse occasions they difficulty logging on. I invite them to new campaigns, using there user names they follow the link and it wont let them o log in with user name instead it insists they create an account when they already have one. Its reaching a point of frustration with some of them that they wont use obsidian portal.
found out what the issue was.
Its the little Xi n the corner to go to log on with user name, its very hard to spot especialy on mobile devices, finding it was a real aggravation. Could it be made more obviouse