Network > QoS > User Groups, click Add to add a new range of client devices. Select Subnet in Subnet / IP Address, and enter the network address (e.g. 192.168.5.0) and adjust the mask if necessary. Select either Staff or Guest as the Group (the Manager group can’t be limited using QoS). In your case Guest would seem appropriate. Save.
Network > QoS > Bandwidth Control select the Enable checkbox in the Individual Bandwidth Limit section. Set your preferred limits for the group you chose above. Save and Apply Changes.
Any client devices assigned to the bandwidth limited group will be limited in upload and/or download speeds as configured. This applies to all devices in the configured group (Staff or Guest). If you’ve assigned additional ranges to the same group, they will also be bandwidth limited.
EDIT: The Individual Limit is NOT a limit per user. The limit is applied to any proscribed single user, as configured. If you configure your limits as 40Mbps down and 10Mbps up, no proscribed user will be able to use more than the configured limits. If you have three simultaneous users in the proscribed group, they will be “fighting” over the 40Mbps limit amongst themselves while the other users not in the group will have the remaining 10Mbps.