Users usually don't have any extra symbol after their names. Only moderators have the diamond symbol. But today I saw a golden dot after a name on a close banner of a question.
What is this and who will have that circle after his/her name?
This is a golden tag badge! Its owners may cast immediate close votes, just like moderators, on questions with the specific tag. This only applies to duplicate-votes at the moment, though.
Currently, there are only two tags where golden tag badges exist, which are command-line and virtualbox. And of these badge types, only 4 badges were ever awarded on Ask Ubuntu in total yet (Oct 2015)!
Their owners are:
Reference for this:
Increase close vote weight for gold tag badge holders (Meta StackExchange)