Tags are added at the time a question is asked and, on occasion, when an answer suggests a tag. AFAIK, the original proposer (OP) of a question can add whatever new tag they need (although the site strongly encourages using existing tags when they suffice). In this fashion tags develop organically to reflect the interests of the site's users. If a subcommunity of users interested in a particular topic appears, appropriate tags ought to emerge automatically from the questions they ask and the answers that are given.
Comments indicate a user needs 300+ reputation to create a new tag. I have therefore created tags for
traminer and wrote wiki entries to distinguish
A good method for low-rep users to create tags is to flag their questions for moderator attention: the flagging dialog allows a couple hundred characters to make a request. Another method is to start a new thread here in meta, but IMHO that's best when the tag might need community discussion.
Low-rep and new users can also provisionally edit tag wikis themselves: those will be reviewed by higher-rep users or moderators for approval. Because even the highest-rep users will not necessarily have the knowledge to write wikis, we welcome and encourage people to write and edit wikis for subjects in which they have expert knowledge.