Roth IRAs were introduced in 1998 and created by the Taxpayer Relief Act of 1997. They have never in any way been linked to an employer (Hence, the Individual in Individual Retirement Account). If this was at any time linked to an employer, you may be thinking of a Roth 401(k) or a regular 401(k) which are both employer sponsored plans.
Assuming you mean a Roth IRA, you can think of it as a container in which you can put investments. A Roth IRA could contain anything from a safe money market account to a mutual fund, to a risky stock; that’s entirely up to you. Most of the major investment houses have lower minimum investments if it’s in an IRA (Vanguard, Fidelidy, etc.). Some offer waived minimums if you contribute a small amount each month, as you mentioned. However, if you already have an IRA account somewhere, there is typically no fee to make any number of contributions that you would like. Just make sure you don’t exceed the maximum, which is $5000/yr if you are 49 or younger, and $6000 if you are 50 or older. The only other requirement is you can only contribute up to the amount of earned income that you have for that year. So, as long as you make more than $5000 in a year, you can contribute the whole $5000 to your Roth IRA. There are penalties for withdrawing from your Roth IRA if you have not yet reached retirement age, however there are a few exceptions.