Sounds like you need a related table 'Donations' with a one to many relationship back to your members (one member, many donations)
Set up a new table, with (at least) the following fields:
fkID - this will be your link field to Members ID
then in Manage | Database | Relationships set up a relationship such that Members::ID = Donations::fkID, allow creation of new records on the Donations table side
On your Members layout, you can now add a portal to Donations, and add Donations as they pour in...