Database design - Members & companies represented by members
Why would you split into User and Account? 

What is Group standing for? It's relating to Account and Company now, that I'm thinking to GroupType or separated tables AccountGroup and CompanyGroup

Above all, based on your current requirements, following my suggestions:

https://dbdiagram.io/d/5d6f2704ced98361d6de2d0d

a user can be part of multiple accounts and have different roles (admin or not)


  Tables: User (Id), Account (Id) and UserAccount (UserId, AccountId)


an account can have one or many groups


  Tables: Group (Id) and AccountGroup (AccountId, GroupId)


each group can have one or many members / a member can be part of many groups


  Tables: Member (Id) and MemberGroup (MemberId, GroupId)


a member can be either an user or a legal entity (company)


  Tables: Company (Id, MemberId), User (add MemberId)


if it's a company, then the company is represented by a user / a user can represent 0,1 or many companies in different groups


  Tables: Company (add RepresentUserId), Group (add RepresentUserId)


Please noted that this is for an idea, not a fullfill solution. I hope this help you much.