The best way is for them to have a broking account with you as third party giving instructions. You got to be careful if you are giving them investment advice. If it goes wrong they could sue you.
NO, if you give advice, especially for compensation, you must be registered.
If you had them all open brokerage accounts and have give you access, this is illegal. When they open the account they define who is to have access. For you to have access is one thing but for you to have discretionary authority is something else which, unless you had an agreement on file with the firm, would be illegal.
No you can not accept deposits and use the money as if it were you own - also illegal