New Users being Created – Can't work out how!

I’m a new administrator for a wordpress hosted site I’m fairly new to wordpress and the other other administrators wouldn’t know a lot either – having ‘inherited’ the site.

I found 4 users recently created as ‘subscribers’ – but cannot find how this is happening – they aren’t newsletter subscribers and don’t appear to be created via the forum – the site has no obvious ‘login/signup’ page either. The email addresses suggest people that would have no obvious interest in the site (we’re an Irish charity – the email addresses are .pl)

The email addresses aren’t familiar to anyone who has worked on the site.

Any ideas? Could our site be getting hacked?

Can someone offer advice because I can’t see how someone could create users on our site without some kind of access?

I do see the feedburner widget in the primary widget area, but as far as I can tell (and as I say I’m coming to this site without too much information as the people who originally set it up are ‘long gone’ I think) the sidebar is being overridden by a template – we have a ‘subscribe’ box – but this a mailchimp newsletter.

To be safe I’ve changed the admin passwords and deleted the new users.

the site in question is mga-charity.ie

I’m most concerned about the fact people are able to create users and I don’t know how!

Many thanks!
Clare.