I would be great to be able to allow site or network Guest Members based on their Membership in other sites or networks.
So lets say that you do not mix a membership and have to handle complicated interactions between various types of members.
Imagine three BuddyPress Networks.
1) Politicians - They create a custom profile and can interact with each other based on their BuddyPress network set up.
2) Public / Voters - They register in their own network and are grouped by their multiple voter districts. (National, State or Province, District, City, etc...)
3) Groups & Organizations - They get sites to promote their interests and needs.
Each type of member can only join the group they match but because they are a member of of one of the three groups they can post comments on the other two and might have other assignable benefits.
So the setup would be done with a plugin in each group.
Guest Member Plugin Settings:
Click Button - Add Guest Member Group
- Choose - Members for Other WP Install: _________________
Now you could go further and only allow members from a certain level. This way you could allow only members for the Public / Voters Network that have a paid membership access to the other networks. Good for making money and controlling spam.