How can I upload email addresses to allow people to track my progress

The simplest solution would be creating a mailing list (you'll find plenty of sites offering such service, if you do not have your own mail server), and importing all those email addresses to the mailing list. You then only need to add the mailing list address to LiveTrack.

Live Track alone has the limit of 25 addresses, so you cannot use it directly for a bigger number of subscribers anyway.

Garmin Connect App LiveTrack Invite Limit | Garmin Customer Support

So if I’m limited to 25 and the mailing list option won’t work what would? 

Mailing list is a single email address, so no - the limit of 25 addresses has no impact on it. And a mailing list itself is typically not limited at all - it may contain millions of addresses (depends on the server, or on the provider).

Tru - could you suggest a mailing list your proposing, as I think I;m confusing this with a list in my address book? Which I don;t think that's what you mean.. thanks in advance

You best use a mailing list implemented by your Internet Service Provider, or by your Email service. You can easily google mail list service at common email providers - for example Gmail: How to Make a Mailing List in Gmail: 12 Steps (with Pictures) (wikihow.com), or just google "mailing list service" 

I'm seriously not getting your suggestion. I have looked at 2 soltions, the one your prosed and Sendinblue, neither proivide an email address after you create a campaign, so you cannot add to the garmin connect.

It is hard to find a free service that does what you want.

I suggest you post the LiveTrack link on Twitter, Facebook group, blog or some place 

You can use IFTTT to automatically post to Twitter from an email: https://ifttt.com/connect/email/twitter

Thanks for trying to fix, I'm doing a chairty ride from London to Paris and have over 60 people who have finded me directly, so was hoping to make this easier, not all are on FB & Twitter! I'm trying to send the live feeds daily (over 4 days)

Is there a way to have the same garmin URL for 4 days (I suspect each session is unique)

Youtube - is it a unique link every day or easier to set up to a single channel and send that out?

Do you have any crew/support that can help you? Do you have a blog about your ride? Then the crew can get the link and post it to the blog. The LiveTrack can be started in advance in the app so there is lots of time to post the link before each ride if you don't have any crew to help you.

The link is unique. 

No idea about Youtube.