No mapping exists from object type WebMatrix.Data.DynamicRecord to a knownmanaged provider native ty

Looks like your issue is with your

groupID

var listOfGroups = db.Query("SELECt GroupID FROM Membership WHERe UserID = (SELECt UserID from Users WHERe Username = @0 )",  username);

Your

listOfGroups

WebMatrix.Data.DynamicRecord

int

groupID

foreach(var groupID in listOfGroups)

Try instead replacing the last line:

AND    e.event_end <= @5", groupID, start, end, groupID, start, end

with:

AND    e.event_end <= @5", (int)groupID.GroupID, start, end, (int)groupID.GroupID, start, end

This should pull the value from the first (and only) column of each dynamic
query result, that value being the int group ID that you are after.

EDIT: Corrected value call syntax of DynamicRecord query result.