Hi oasix,
Bandwidth restrictions are handled in the role. So you need to figure out which role is used for your guests. For that role, you can define bandwidth limitations:
Those limits can be defined on a per-role basis, which means, that all clients with role will get the amount of bandwidth, or per-user, which means, that every user with this role will get this amount of bandwidth or on a per-ap-group basis, which means, that all clients connected to the same AP group will get this amount of bandwidth.
BR
Florian