The good news is that LinkedIn has re-instated the ability to send Messages to your contacts selected via your tags. The limit on the number of recipients of your Message does however remain at 50.
Since April I have been in a discussion thread on the LinkedIn Help Forum that has been debating/lobbying about the removal of the ability to select contacts to send messages by using the tags in your Contacts. Today, the issue was flagged as having been fixed.
From your top level menu select Network > Contacts.
Then Select Tags > and the tagged group of Contacts that you want to Message.
You have the option to Select All or, to select individuals from the tagged group. In this example I have selected a tag that picks the fellow members of The Social Media Bureau.
After making your selection then click on the Message option. You can adjust which one of your email addresses to use rather than your Primary Email.
LinkedIn does set as a default that all recipients can see each other and their email addresses. In an earlier post I did advise that perhaps this option should be turned off.
Note that the option of selecting by company, job title, industry etc. are all still available.