Yes.  It is in fact the only requirement to be a Legion Rider.  You must be a member in good standing..
It does not cost anything extra to join the Legion Riders.  If your dues are up to date for the Legion that is all you must pay.  Of course, swag, vests, motorcycles cost money but those are choices not requirements
The Legion riders are neither a M/C or an R/C.  The are a unit of the Legion, like the colour party.  You must be a Legion member to participate. 

We strive to get along with everyone as veterans and their families come from everywhere.  

We are a group that is here for Veterans and Legion Members. 
No, there is no minimum level.  We would hope you attend meetings and participate in Legion and charity events.  We are hosts of the Annual Ride for Hospice now and volunteers are essential for success.  
By participating you can make new friend, get out of the house, go for motorcycle rides with one person or the group.  There is so much to gain by participating but you odn't have to.