No, a tag is just a tag: a list of strings separated by a comma: so just a new field in the DB for the survey table. Surveys have tags: you can add or remove a tag to a survey, and you can make an union of all the surveys having this or this tag. So you can "group" surveys by union: all the surveys having the same tag... But there is nothing like an entity "group".
A real "group" is different: a group is a relation: at least two new tables in the DB, one table for the groups (group name, description, etc), and then another table for the relation (this survey belong to this group). Groups have surveys, you can add or remove a survey to a group, then you can show all the surveys belonging to a group. But in general, the reason why you use a new entity like a "group" is not only to filter items, it's rather to create
an advanced permission system
. Group of users ("roles") can do things with a group of items ("categories"). Eg: Users with role "student" can edit any survey in category "university" but can't edit survey in category "laboratory".
Implementing an ACL system inside LimeSurvey is a long term goal, because it would imply to rewrite the existing permission system.
But the tag system, that could be done quite quickly.