Can you run this:

EXECUTE AS LOGIN = 'Domain\grouplogin'

go

SELECT u.name, u.type_desc, CASE WHEN u.type = 'G' THEN is_member(u.name) END AS ismeber

FROM sys.server_principals u

JOIN sys.server_role_members rm ON u.principal_id = rm.member_principal_id

JOIN sys.server_principals r ON r.principal_id = rm.role_principal_id

WHERE r.name = 'sysadmin'

go

REVERT