There are multiple ways that you can add tags.
First you need to create your tags by going to Settings>Tags.
If you are wanting to assign an individual person or a small group of people a tag the fastest way to do this is to go to their profile pages. Here, located under their profile picture, you will see add/edit tags
If you are wanting to assign a tag to a large group I would suggest doing a bulk action.
To do this you would go to Reports and run a Listing of All..(Members, Prospects, Trial Members, Inactive, Former) report.
Check the boxes next to the individuals you are wanting to be in this tag group and then scroll to the bottom.
You will see where it says Bulk Actions.
Next to this are two drop down boxes. In the first select Assign a Tag and in the second select the desired tag. Then hit Go.
I hope this was helpful:)
Thank you for being a RainMaker!!