A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
Dear Sam C,
Good day!!
I understand your concern and to my knowledge, I am sorry to convey that there is no out of the box feature or setting in SharePoint Online to setup a user license agreement when access the SharePoint Service.
I agree with you having the ability to setup the user license agreement for the users when accessing the SharePoint Online service for the first time via SharePoint Online admin center will benefit the users and I suggest you to add your valuable idea as feedback in the SharePoint · Community (microsoft.com) which is the best place to share our ideas and improve the Microsoft products.
However, during my search I believe you can achieve your requirement with the help of the Conditional Access in the Microsoft Entra. So, I request you give a shot by posting your concern in the related community: Questions - Microsoft Q&A with Microsoft Entra tag to get the detailed help from the experts.
Apologies for redirecting you to different community as the members in the category posted focus on the users with out of the box concerns on Microsoft 365, and have limited knowledge on the Microsoft Entra; so to get the fast and better assistance, we have redirected you in the correct path.
Appreciate your patience and understanding. Have a great day!!
Best Regards,
Sophia