Very useful thread. A few more info :

Cinematik
There are not official rules (like those mentioned ...Registered > 8Weeks , Upload > 100Gb , Ratio > 1.15)
May be there were in the past, but I do not remember.
Now, any user can request invites at certain thread at the forum. And the only rule needed is that the user has to be active in a lot of ways.
A Power user rank indicates some action only. Many power users with very good ratio and even some TB upload are rejected invites, because they are not active enough.
Active means, besides ratio : Torrent comments, forum posts, upload movies and not hit and runs.

Greek-Team
2 invites can be exchanged indeed for 2000 bonus points but we have to know that ..... Only VIP and above can send invites !!
Meaning that many users have a lot of invites, but they can't send them as they are not VIP. And VIP means to donate only.

GreekLeech
Silver Rank do not get invites. Only when the are promoted to the next rank, Gold, they get one.
The whole invite process is the folowing:

Gold users : 1 invite (100Gb , Ratio >1.5 , Register >90 days)
Jade users : 2 invites (250Gb , Ratio >2 , Register >150 days)
Coral users : 2 invites (500Gb , Ratio >2.25 , Register >180 days)
Topaz users : 2 invites (1TB , Ratio >2.5 , Register >210 days)
Emerald users : 3 invites (3TB , Ratio >3 , Register >250 days)
Ruby users : 3 invites (5TB , Ratio >3 , Register >300 days)
Diamond users : 5 invites (10TB , Ratio >3 , Register >360 days)

Note that invites are not added (so f.e. diamond users do not have 1+2+2+2+3+3+5. They only get 5! )

Besides, other ranks that can't be reached by users and only appointed by staff :

Uploader : 2 invites
Trusted+ : 3 invites
Trusted+ : 3 invites
VIP : 3 invites
VIP+ : 10 invites (the maximum that someone can get)
Translator : 2 invites
GrL radio DJ : 2 invites
GL Addicted+ : 10 invites (the maximum that someone can get)


kapaki.info
Invites can be exchanged by bonus points
2 Invites = 350 points

My two cents