Re: Adding some text to the signup form?
Create a child theme (it’s easy — do a search for instructions on how), copy register.php from /plugins/buddypress/bp-themes/bp-default/registration to /registration in your new child theme (which will just be the default theme plus this change and any other changes you make in future). Edit that registration.php.
Now your new registration page will override the default, and you are safe from having your changes overwritten by future version upgrades!
Any time you want to override anything (particularly styles), you can just copy files from bp-default, preserving folder structure, to your child theme and modify them there. It’s a little confusing at first, but is extremely powerful and useful.