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Yes, if you have your site set to allow external users, they can view and edit information in a web database. Just be aware that at this moment, they must have Microsoft E-mail ( hotmail.com) or their own MOS account.
Hope this helps
Based on your description, I assume that you are using the Small Business version of Office 365. I would like you to follow these steps to invite external users.
1. Please select the “Site Settings” item from “Site Actions” drop-down menu in the “Team Site” page.
2. Click the “Go to top level site settings” button in this page.
3. Click the “Site collection features” button in this page.
4. Please make sure the "External user invitations" item status is "Active".
5. Please select the “Share Site” item from “Site Actions” drop-down menu in the “Team Site” page.
6. Please fill in the customer’s account（email address for Hotmail or others Microsoft Online Services IDs） who you want to invite in the “Visitors” or “Members”. After that, please click the “Share” button.
7. The customer who has been invited will receive an E-mail like the following picture, they can click the “Accept your invitation!” button to access your SharePoint site.
Yes, the limit is 50.
Where do I go to allow extrenal users, and where can I read about giving permission to externall users.
Is there a limit on the amount of external users that I can have?
I don't forsee using 50 anytime soon, but if I ever need more, is there a way to get more invites?
At this time there is no offering to be able to buy more that I have seen.