Hi Padraig and welcome!
I think you are confusing G2G and WikiTree membership. For G2G, you are here and posting. Anyone can. For WikiTree, you are logged in as a Guest. You have to complete your membership by volunteering, adding some interest tags, and signing the Honor Code. You should get instructions from WikiTree. Or hover your mouse over the Help menu upper right, and click Intro to WikiTree to find the steps.
For G2G purposes, comments are one of two ways to respond to questions. Tags are things you use to attract attention from interested parties. When you post a G2G question, keep going down the page until you run out of green. At the bottom of the form is a place for tags, usually surnames or keywords like 'g2g,' 'help,' 'irish_roots,' etc. You can edit your question to add this stuff after posting.