Internal: the Teams guest work and people's addresses ({handle}.magpie.diy) were built on different computers. Merging them, the handle hint on the team invitation page and in the app's team invitation screen still allowed underscores, which handles no longer take; both now say "Use lowercase letters, numbers and hyphens." UsernameService::suggest() also skips a handle whose address an older handle with an underscore already has, as claim() refuses it.
Team invitations: handles without underscores
Internal: the team invitation page and app screen describe handles as letters, numbers and hyphens, and a suggested handle avoids an older underscore handle's address.