the options for inviting someone to a chatroom are either by matrix ID (if they already have one), or by email (where they will get an email invite that should resolve to a matrix invite once they sign up, although it's quite fragile), or by linking the room via matrix.to - e.g. "come to matrix.to/#/#matrix:matrix.org". matrix.to should work pretty well; what problems did you see? is this to an invite-only room or a public chatroom?