To answer your question, there is no limit on the size of colwiz Groups. To create your closed group from your colwiz Web home page navigate to:
Groups (left-hand panel) > Create Group > Invite People to this Group (blue button on right-hand side panel)
If you have an email list already compiled, you can simply cut and paste this list directly into the pop-up box that appears upon clicking ‘Invite People to this Group’. This box also displays an option to add contacts from your colwiz address book. You can populate your address book as follows:
colwiz Web home page > Contacts (left-hand panel) > Invite your Colleagues
Note: Adding friends through your social networks or by email is a great way to increase the storage capacity for your colwiz account, for free.