Trustees have signatory authority over bank accounts and investments.
You can be – 1) participant and a trustee, 2) just a participant or 3) just a trustee.
Only a participant can rollover funds and make contributions. You can participate in the Solo 401k plan at any point you are an employee in the business adopting the Solo 401k. It is strongly recommended that if you and your spouse work together in the business adopting the Solo 401k, that you both be participants. It is generally recommended you’re not a trustee unless you’re also a participant in the plan.
For some investments, only the trustee can sign the investment documents. The trustee is also the signing authority on any bank accounts opened in the name of the 401k trust. If you (and your spouse) want to make an investment that requires trustee signatures, you may want to consider both being trustees of the plan.
For Solo 401k accountholders where your spouse is involved, many spouses begin as co-participants and elect one person in the household to be the plan trustee. A second trustee can always be added later on.