Board Thread:The Quarters - Q & A/@comment-2601:206:8300:19A0:4835:D75D:55D:905E-20180704154840/@comment-25085701-20180704192934

Missing a couple of words here. Are you asking whether you can be invited if you already have a Habitica account, or if you already have a party?

If it's the former, you just need to give them your User ID. If it's the latter, you can only be in one party and would need to leave it before you get invited to a new one.