1) OG - ADD GUEST OPTIONS Show checkboxes : Yes
In this case, a user who is member of several groups only appears the first time he is encountered
For example, if I have
group1: user1, user2,
Group2: user1, user3
Group3: user1, user2, user3

RSVP will display
group1
user1
user2
group2
user3
group3
(no users)

2) OG - ADD GUEST OPTIONS Show checkboxes : No
Everything is displayed correctly, however, only full groups can be added. Selecting one member always fails with the following message:
"The following guests could not be added, because they have been added earlier:
•(name of the organizer)"

Actually, only adding the organizer himself works.

Please apologize for grouping 2 different issues, but I guess this should be easily (hopefully) solved all at once