Yes, Members can register as a group with other Members provided they all qualify for the relevant ballot. For example, a Silver Member with a valid Silver Membership will be able to register for the Silver ballot together with other Silver Members.

If Members register as a group, will each ballot entry either be successful or unsuccessful, or can a ballot entry be partially successful?

Members who register for the ballot as a group will be balloted together and the whole group will either be successful or unsuccessful. There will be no partial success whereby one Member of the group is successful while another is unsuccessful or vice versa. 

If Members register with a group does this decrease their chances of success?  

This depends on how many ballot applications there are in total and the grouping of tickets remaining as the ballot is run. For example, if there are 20 seats left and they are all pairs, any groups registered as threes and fours would not be drawn out, only pairs and singles. The process would continue until all seats are filled.