SMS Campaigns - setting up groups/lists, service provider recommendations

I have a number of opt-in Groups/Lists who receive weekly text message reminders based on the day of the week, or the day of the month. (1) Who do you recommend for third-party providers?  (2) How do you set up the individual lists based on the submission form values?

I'd like to integrate with Marketo from Landing Page form submission through to Group/List assignation which would then automatically subscribe the user to the appropriate ongoing campaign based on their form.