Another variation on the previous suggestions. See what happens to all the other solutions when any of the roles is changed into for example 'R&D'. Plus, I don't like the replace()-approach to get rid of the last ','. I rather don't generate it if it's not needed instead of taking it off in an additional processing step:

SELECT

u1.UserID,

(

select case row_number() over (order by (select 1)) when 1 then '' else ', ' end

+ t.RoleName AS [text()]

from (

select row_number() over (order by min(u2.RoleAssignedDate)) as nr, u2.RoleName

from @UserRole u2

where u2.UserID = u1.UserID

group by u2.RoleName

) t

order by t.nr

for xml path(''), type

).value('.', 'nvarchar(max)') as Roles

from (

select

UserID

from @UserRole

group by

UserID

) u1

order by u1.UserID