The "login-group" command:
or: fossil login-group join REPO [-name NAME]
or: fossil login-group leave
With no arguments, this command shows the login-group to which the repository belongs.
The "join" command adds this repository to login group to which REPO belongs, or creates a new login group between itself and REPO if REPO does not already belong to a login-group. When creating a new login- group, the name of the new group is determined by the "--name" option.
The "leave" command takes the repository out of whatever login group it is currently a part of.
About Login Groups:
A login-group is a set of repositories that share user credentials. If a user is logged into one member of the group, then that user can access any other group member as long as they have an entry in the USER table of that member. If a user changes their password using web interface, their password is also automatically changed in every other member of the login group.