To play Halo Reach with a guest on Xbox Live, one person must have an Xbox Live gold membership. The second person has to sign in under the guest profile (you can have up to three guests per profile)
yes, you can play DLC that was downloaded on a Xbox LIVE profile and play it on a non Xbox LIVE profile
yes.
You cant sell a Xbox live profile.
Yes, just select the profile and choose "Join Xbox Live." You may have to change your gamertag, but your Gamerscore will stay the same.
The number next to your profile is the number of years that profile has been on xbox live.
To get a Xbox live membership, you need to register. Your profile must have been registered to go online.
You can restart it then try to sign in again.
If you have a Xbox live Gamer profile, you can just recover it on the new Xbox. If you are not online, you will need to buy a transfer cable.
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No
One person may be signed on per controller, but not with the same profile. If you need someone else to be signed on to play xbox live, many games have a "guest" option that allow a controller to be signed on as a live account's guest.