Hello:
1) The setup for a PM client Community member should be part of the User's account setup. Then there is only one PM password to remember. It is not User friendly to treat the Community forums as a separate enitity except for a Community User name (i.e. not show the User's real name) for PM clients.
2) If the intent is to allow non-PM clients (prospective client) the ability to ask questions via PM Commuity forums prior to their decision to go with PM then they could be flagged as non-PM clients. If they become a PM Client then their Community user name (non-PM Client) could be linked to their PM-Client account when they setup their PM client account since their email used during the non-PM client could be the link between the two. Alternatively they could just select a new Community user name.
3) It is highly doubltful that any non-PM client would be a continuous PM Community forum member. So the focus should be making it simple for PM-Clients.
Cheers